chore(deps): update dependency google-cloud-spanner to v1.15.0 (#3166)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| [google-cloud-spanner](https://github.com/googleapis/python-spanner) | minor | `==1.13.0` -> `==1.15.0` |

---

### Release Notes

<details>
<summary>googleapis/python-spanner</summary>

### [`v1.15.0`](https://github.com/googleapis/python-spanner/blob/master/CHANGELOG.md#&#8203;1150httpswwwgithubcomgoogleapispython-spannercomparev1140v1150-2020-03-17)

[Compare Source](https://github.com/googleapis/python-spanner/compare/v1.14.0...v1.15.0)

##### Features

-   Add emulator support ([#&#8203;14](https://www.github.com/googleapis/python-spanner/issues/14)) ([b315593](https://www.github.com/googleapis/python-spanner/commit/b315593bd3e473d96cc3033f5bbf0da7487e38eb))
-   Export transaction.\_rolled_back as transaction.rolled_back ([#&#8203;16](https://www.github.com/googleapis/python-spanner/issues/16)) ([974ee92](https://www.github.com/googleapis/python-spanner/commit/974ee925df1962f559d6cb43318ee301e330e8f2))
-   Add support for backups ([#&#8203;35](https://www.github.com/googleapis/python-spanner/issues/35)) ([39288e7](https://www.github.com/googleapis/python-spanner/commit/39288e784826c5accca71096be11f99ad7f930f4))
-   Implement query options versioning support ([#&#8203;30](https://www.github.com/googleapis/python-spanner/issues/30)) ([5147921](https://www.github.com/googleapis/python-spanner/commit/514792151c2fe4fc7a6cf4ad0dd141c9090a634b))

##### Bug Fixes

-   Remove erroneous timeouts for batch_create_session calls ([#&#8203;18](https://www.github.com/googleapis/python-spanner/issues/18)) ([997a034](https://www.github.com/googleapis/python-spanner/commit/997a03477b07ec39c718480d9bfe729404bf5748))

### [`v1.14.0`](https://github.com/googleapis/python-spanner/blob/master/CHANGELOG.md#&#8203;1150httpswwwgithubcomgoogleapispython-spannercomparev1140v1150-2020-03-17)

[Compare Source](https://github.com/googleapis/python-spanner/compare/v1.13.0...v1.14.0)

##### Features

-   Add emulator support ([#&#8203;14](https://www.github.com/googleapis/python-spanner/issues/14)) ([b315593](https://www.github.com/googleapis/python-spanner/commit/b315593bd3e473d96cc3033f5bbf0da7487e38eb))
-   Export transaction.\_rolled_back as transaction.rolled_back ([#&#8203;16](https://www.github.com/googleapis/python-spanner/issues/16)) ([974ee92](https://www.github.com/googleapis/python-spanner/commit/974ee925df1962f559d6cb43318ee301e330e8f2))
-   Add support for backups ([#&#8203;35](https://www.github.com/googleapis/python-spanner/issues/35)) ([39288e7](https://www.github.com/googleapis/python-spanner/commit/39288e784826c5accca71096be11f99ad7f930f4))
-   Implement query options versioning support ([#&#8203;30](https://www.github.com/googleapis/python-spanner/issues/30)) ([5147921](https://www.github.com/googleapis/python-spanner/commit/514792151c2fe4fc7a6cf4ad0dd141c9090a634b))

##### Bug Fixes

-   Remove erroneous timeouts for batch_create_session calls ([#&#8203;18](https://www.github.com/googleapis/python-spanner/issues/18)) ([997a034](https://www.github.com/googleapis/python-spanner/commit/997a03477b07ec39c718480d9bfe729404bf5748))

</details>

---

### Renovate configuration

:date: **Schedule**: At any time (no schedule defined).

:vertical_traffic_light: **Automerge**: Disabled by config. Please merge this manually once you are satisfied.

:recycle: **Rebasing**: Never, or you tick the rebase/retry checkbox.

:no_bell: **Ignore**: Close this PR and you won't be reminded about this update again.

---

 - [ ] <!-- rebase-check -->If you want to rebase/retry this PR, check this box

---

This PR has been generated by [WhiteSource Renovate](https://renovate.whitesourcesoftware.com). View repository job log [here](https://app.renovatebot.com/dashboard#GoogleCloudPlatform/python-docs-samples).

Author: renovate-bot

appengine/standard_python37/spanner/requirements.txt

@@ -1,2 +1,2 @@
-google-cloud-spanner==1.13.0
+google-cloud-spanner==1.15.0
 Flask==1.1.1

functions/spanner/requirements.txt

@@ -1 +1 @@
-google-cloud-spanner==1.13.0
+google-cloud-spanner==1.15.0