Support latest babel 7 only

[danez]

This drops babel 6 as babel 7 now has also different package names which makes it rather complicated to support both.

This fixes several issues:

• The plugins received from OptionManager are now classes and not arrays and have a prop options
• The plugin key that the resolver was checking in the babel 7 case was wrong it is now module-resolver see here https://github.com/tleunen/babel-plugin-module-resolver/blob/master/src/index.js#L25
• The tests are broken, because jest by default sets the env to "test" which was interfering with the test that tested different environments.
• The babel internal test does not work as internals are not exposed this way anymore

This is basically the same as #76 with fixed tests.

[danez]

This is needed for jest and installs the babel-core bridge. https://github.com/babel/babel-bridge
We could have used the bridge here, but I think creating a new major version is not that big of a problem here, than it is in jest.

[tleunen]

Oh nice! I noticed that issue with jest when I worked on that in #76

Thank you so much for fixing it.

[tleunen]

What do you suggest here? Moving @babel/core from peerDep to dep? Or releasing a new major version?

Since it's still a beta, most users are probably still using babel 6 and I'm guessing this might create more issues for them?

Otherwise, I'm all good for this PR, thank you.

[danez]

I'm not sure. I think I would release 4.0 soon-ish, as it is needed for the babel plugin 3.x?

To not break more users i think 4.0 should be released as is. Or with babel 7 removed as it is only working with some older babel 7 version anyway. If not removed I would only fix the key check for babel 7, as it is obviously wrong now that the babel plugin sets its name here.

But I guess I would prefer releasing 4.0 without babel 7 and then do 5.x-beta for babel 7.

Complete other solution would be to support both, same as jest. But then users of babel 7 have to install @babel/core@7 and babel-core@7.0.0-bridge.0. see https://facebook.github.io/jest/docs/en/getting-started.html#using-babel

[tleunen]

Yes good idea. I'll release v4 with Babel 6, then v5 with Babel 7.

Forcing the users to install 7 and the babel-core bridge is not a viable solution imo. It's a bit an ugly hack :/

(I'll actually finally try to setup these 2 packages with lerna and keep the versions synchronized to make it easier for all users)