Use rollup for build system

[paularmstrong]

Problem:
As discovered in #78, the build system needs to be updated in order to unlock updating to React v16.x.

Solution: Mostly copies the changes from #76 for rollup. Upgrades babel & mocha.

Using rollup for react-globalize cuts 2.02KB (gzipped) off the production vendor.js file in Twitter Lite 👍

This will create all of the same formats for builds as #76: cjs, umd, and esm.

[paularmstrong]

FYI: the upgrade to React 16 is ready to go once this is merged: paularmstrong@1ef481e. Once that's done, we could go forward with updating to use more ES6, new eslint/prettier/whatever.

[rxaviers]

Thank you.

Solution: Mostly copies the changes from #76 for rollup. Upgrades babel & mocha.

For that reason, I've cherry-picked the original changes from that branch and applied yours on top of it in an attempt to be fair to both of you @paularmstrong and @diligiant.

Merged by 8d92b0f and dd35eb8.

[rxaviers]

Using rollup for react-globalize cuts 2.02KB (gzipped) off the production vendor.js file in Twitter Lite 👍

This is awesome

[rxaviers]

FYI: the upgrade to React 16 is ready to go once this is merged: paularmstrong/react-globalize@1ef481e. Once that's done, we could go forward with updating to use more ES6, new eslint/prettier/whatever.

@paularmstrong please can you submit a PR