E2E test Babel with itself before publishing

[nicolo-ribaudo]

Q	A
Fixed Issues?	Fixes #6134
Tests Added + Pass?	Yes
License	MIT

This PR adds a CircleCI workflow which

1. Publishes Babel on an internal verdaccio registry
2. Upgrades the @babel/* versions used by our repo, so that it uses the new "published" version
3. Builds Babel and runs the tests

I created the workflow as two separate jobs so that if we decide to add other e2e tests (e.g. create-react-app, istanbul, ...) we can run them in parallel after "publishing" Babel only once.

I initially planned to only run these tests using a cronjob once a day, but since it only takes 7 minutes and it's run in parallel with all the other Travis/Circle tests, we could always run it.
This will probably need to change when we add other e2e tests.

The Introduce a bug commit is an example of a bug which isn't caught by our unit tests but which fails the e2e test 😄

[buildsize]

File name	Previous Size	New Size	Change
babel-preset-env.js	2.78 MB	2.78 MB	9 bytes (0%)
babel-preset-env.min.js	1.67 MB	1.67 MB	9 bytes (0%)
babel.js	2.96 MB	2.96 MB	9 bytes (0%)
babel.min.js	1.63 MB	1.63 MB	9 bytes (0%)
test262.tap	9.86 MB	[deleted]

[JLHwung]

LGTM on the ci config and integration test scripts.

[existentialism]

❤️