Migrate to Babel 7

[leebyron]

Following React's move to Babel v7, GraphQL.js should do the same. The babel config for this project is not that complex, so hopefully this is an easy transition.

[leebyron]

Something to consider is Babel v7 may not build correctly on Travis CI's Node v4 subjob. Technically we don't care about Babel builds on Node v4, but we do want the npm distributed bundle to be Node v4 safe - at least through the LTS window (end of April)

We could either:

• drop the CI job early and keep our fingers crossed and eyes open,
• wait to do this migration until May
• write a slightly complex CI job that uses a different version of node to run the build than run the tests on the output

[AaronCCWong]

Is this waiting on babel v7 to come out of beta? If not, I'd like to take a stab at it.

[IvanGoncharov]

Implemented in #1350 by @AaronCCWong 👍

[leebyron]

Great work, @AaronCCWong!