Migrate to ES modules

[vlad-zhukov]

Closes #27

Current master:
22465 rambda.cjs.js
21044 rambda.esm.js
12483 rambda.umd.js

This PR:
18633 rambda.cjs.js
17541 rambda.esm.js
10932 rambda.umd.js

[vlad-zhukov]

@selfrefactor it looks like you've tried to merge it, but something went wrong due to a conflict? I will fix the PR tomorrow so you can merge it.

Tests are passing in my PR. Jest automatically transpiles all files with Babel if there is a .babelrc file in the root directory, that's why I created it.

As a sidenote Travis runs a very outdated version of yarn (v0.17.8) but I've no idea how to fix it.

[selfrefactor]

Yep, we had the bad luck of working on the same time. I removed some tests that were using async keyword and were causing the build to fail.

I will stop pushing changes till you figure out a solution for this PR to be merged.

As for the outdated yarn on Travis - let's first merge the PR and then we'll think about that.

Once again, thanks again Vlad for the massive PR - it looks like you spend good 1-2 hours on that :)

Let me know if you need something from me in regards of merging this PR(in case that is still posible).

[vlad-zhukov]

It should be good to merge now. We should restore tests back to use async-await and test source files (rambda.js) instead of the CJS bundle.