Forcing babel-loader 7.1 and updating resolve-rc for new argument

[weaverryan]

In babel-loader 7.1, they added a new argument to resolve-rc. This updates Encore to require 7.1 and pass in that new argument.

Of course, the underlying problem is that we're relying on an internal function - so it's entirely valid for them to break BC. We should obviously minimize doing this, but I still think it's a good idea: I don't want to try to re-implement their logic. For example, there are now 2 new places where you can store the Babel configuration... and by using resolve-rc, we're now checking for those automatically.

[weaverryan]

Tests are passing now - the appveyor failures will be fixed via #28

[robfrawley]

@weaverryan This change makes using babel-loader v7.0 incompatible with Encore. There must be a changelog that conveys when breaking changes are introduced through use of dependencies! This is important, as yarn doesn't behave like composer in that if someone explicitly requires a different version than your lock file, it doesn't halt in a conflict. Due to this, there should be a more verbose listing of version changes when breaking changes are introduced.

For example, now (if you are using [email protected]):

  FileWriteStream: 
   { [Function: WriteStream]
     super_: { [Function: Writable] WritableState: [Function: WritableState], super_: [Object] } } }
    at assertPath (path.js:7:11)
    at Object.join (path.js:1211:7)
    at find (/home/rmf/repositories/web-apps/rf/node_modules/babel-loader/lib/resolve-rc.js:18:19)
    at module.exports (/home/rmf/repositories/web-apps/rf/node_modules/babel-loader/lib/resolve-rc.js:35:23)
    at module.exports (/home/rmf/repositories/web-apps/rf/node_modules/@symfony/webpack-encore/lib/config/parse-runtime.js:66:47)
    at Object.<anonymous> (/home/rmf/repositories/web-apps/rf/node_modules/@symfony/webpack-encore/bin/encore.js:18:23)
    at Module._compile (module.js:570:32)
    at Object.Module._extensions..js (module.js:579:10)
    at Module.load (module.js:487:32)
    at tryModuleLoad (module.js:446:12)