Update fs-extra requirement to ^7.0.0 (#219)

Updates the requirements on [fs-extra](https://github.com/jprichardson/node-fs-extra) to permit the latest version.
<details>
<summary>Changelog</summary>

*Sourced from [fs-extra's changelog](https://github.com/jprichardson/node-fs-extra/blob/master/CHANGELOG.md).*

> 7.0.0 / 2018-07-16
> ------------------
> 
> - **BREAKING:** Refine `copy*()` handling of symlinks to properly detect symlinks that point to the same file. ([#582](https://github-redirect.dependabot.com/jprichardson/node-fs-extra/pull/582))
> - Fix bug with copying write-protected directories ([#600](https://github-redirect.dependabot.com/jprichardson/node-fs-extra/pull/600))
> - Universalify `fs.lchmod()` ([#596](https://github-redirect.dependabot.com/jprichardson/node-fs-extra/pull/596))
> - Add `engines` field to `package.json` ([#580](https://github-redirect.dependabot.com/jprichardson/node-fs-extra/pull/580))
> 
> 6.0.1 / 2018-05-09
> ------------------
> 
> - Fix `fs.promises` `ExperimentalWarning` on Node v10.1.0 ([#578](https://github-redirect.dependabot.com/jprichardson/node-fs-extra/pull/578))
> 
> 6.0.0 / 2018-05-01
> ------------------
> 
> - Drop support for Node.js versions 4, 5, & 7 ([#564](https://github-redirect.dependabot.com/jprichardson/node-fs-extra/pull/564))
> - Rewrite `move` to use `fs.rename` where possible ([#549](https://github-redirect.dependabot.com/jprichardson/node-fs-extra/pull/549))
> - Don't convert relative paths to absolute paths for `filter` ([#554](https://github-redirect.dependabot.com/jprichardson/node-fs-extra/pull/554))
> - `copy*`'s behavior when `preserveTimestamps` is `false` has been OS-dependent since 5.0.0, but that's now explicitly noted in the docs ([#563](https://github-redirect.dependabot.com/jprichardson/node-fs-extra/pull/563))
> - Fix subdirectory detection for `copy*` & `move*` ([#541](https://github-redirect.dependabot.com/jprichardson/node-fs-extra/pull/541))
> - Handle case-insensitive paths correctly in `copy*` ([#568](https://github-redirect.dependabot.com/jprichardson/node-fs-extra/pull/568))
> 
> 5.0.0 / 2017-12-11
> ------------------
> 
> Significant refactor of `copy()` & `copySync()`, including breaking changes. No changes to other functions in this release.
> 
> Huge thanks to **[[**manidlou**](https://github.com/manidlou)](https://github.com/manidlou)** for doing most of the work on this release.
> 
> - The `filter` option can no longer be a RegExp (must be a function). This was deprecated since fs-extra v1.0.0. [#512](https://github-redirect.dependabot.com/jprichardson/node-fs-extra/pull/512)
> - `copy()`'s `filter` option can now be a function that returns a Promise. [#518](https://github-redirect.dependabot.com/jprichardson/node-fs-extra/pull/518)
> - `copy()` & `copySync()` now use `fs.copyFile()`/`fs.copyFileSync()` in environments that support it (currently Node 8.5.0+). Older Node versions still get the old implementation. [#505](https://github-redirect.dependabot.com/jprichardson/node-fs-extra/pull/505)
> - Don't allow copying a directory into itself. [#83](https://github-redirect.dependabot.com/jprichardson/node-fs-extra/issues/83)
> - Handle copying between identical files. [#198](https://github-redirect.dependabot.com/jprichardson/node-fs-extra/issues/198)
> - Error out when copying an empty folder to a path that already exists. [#464](https://github-redirect.dependabot.com/jprichardson/node-fs-extra/issues/464)
> - Don't create `dest`'s parent if the `filter` function aborts the `copy()` operation. [#517](https://github-redirect.dependabot.com/jprichardson/node-fs-extra/pull/517)
> - Fix `writeStream` not being closed if there was an error in `copy()`. [#516](https://github-redirect.dependabot.com/jprichardson/node-fs-extra/pull/516)
> 
> 4.0.3 / 2017-12-05
> ------------------
> 
> - Fix wrong `chmod` values in `fs.remove()` [#501](https://github-redirect.dependabot.com/jprichardson/node-fs-extra/pull/501)
> - Fix `TypeError` on systems that don't have some `fs` operations like `lchown` [#520](https://github-redirect.dependabot.com/jprichardson/node-fs-extra/pull/520)
> 
> 4.0.2 / 2017-09-12
> ------------------
> 
> - Added `EOL` option to `writeJson*` & `outputJson*` (via upgrade to jsonfile v4)
> - Added promise support to [`fs.copyFile()`](https://nodejs.org/api/fs.html#fs_fs_copyfile_src_dest_flags_callback) in Node 8.5+
></table> ... (truncated)
</details>
<details>
<summary>Commits</summary>

- See full diff in [compare view](https://github.com/jprichardson/node-fs-extra/commits/7.0.0)
</details>
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Author: dependabot[bot]

package.json

@@ -48,7 +48,7 @@
   },
   "dependencies": {
     "bluebird": "^3.0.6",
-    "fs-extra": "^6.0.0",
+    "fs-extra": "^7.0.0",
     "glob-all": "^3.1.0",
     "is-wsl": "^1.1.0",
     "jszip": "^3.1.0",