Change babel-core to peerDependency for compatibility with Babel 7 (#4162)
Change babel-core to peerDependency for compatibility with Babel 7 (#4162)

* Change babel-core to peerDependency for compatibility with Babel 7

This makes babel-jest compatible with whichever version of Babel is
used in a given project, since babel-core@6 is not necessarily
compatible with plugins made for Babel 7.

Note that this is a breaking change (albeit low impact).

* Revert babel-core upgrade to 7.x

cpojer/jest,facebook/jest,bookman25/jest,ide/jest,ide/jest,facebook/jest,bookman25/jest,ide/jest,skovhus/jest,skovhus/jest,cpojer/jest,skovhus/jest,facebook/jest,cpojer/jest,bookman25/jest