Fix CircleCI builds

[ahutchings]

I saw that CircleCI builds have failed against the last few commits. Jest was failing with a SyntaxError on the import keyword, to it seems the babel transpiling was not working.

Hopefully this fixes it - it adds two dev dependencies required by jest as documented at http://facebook.github.io/jest/docs/getting-started.html#using-babel.

• babel-jest
• babel-polyfill

[tommoor]

Lets give it a try!

[ahutchings]

Looks like it worked! 🎉