move babel-core to peerDependencies

[hulkish]

Make compatible with babel 7

Currently, because babel-core is defined as a regular dependency - it prevents babel 7 usage.

Also, please see:

• https://github.com/facebook/jest/pull/4557/files
• https://github.com/babel/babel-bridge

Just a change to package.json

[codecov-io]

Codecov Report

Merging #20 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master    #20   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           1      1           
  Lines         105    105           
  Branches       24     24           
=====================================
  Hits          105    105

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[kentcdodds]

Thank you! This will be released as a breaking change. Luckily upgrading will be easy :) I really appreciate your help!