Decaffinate tests

[realityking]

This PR does the following changes

• Decaffinate the tests
• Require Node.js 6
• Add eslint for linting

I'll back out the eslint change into its own PR after a first review as the license issue for eslint-config-airbnb-base still exists.

[honzajavorek]

Wow! Thanks 👍

LGTM. My only concern would be that I'm proponent of the reverse approach:

1. Decaf the code, leave tests intact, be sure nothing got broken.
2. Decaf the tests.

However, we have a good past experience with the decaffeination, it's not breaking things, so it probably doesn't matter.

Also, you're right about the airbnb-base license issues.

[honzajavorek]

nitpick: npm test is sufficient

I didn't want to approve technically, but just by words for now. Need to check all licenses etc.

[realityking]

I split off linting into #21. That means no dependency changes in this PR🎉

I decaffeinated the actual parser in #22. Feel free to merge that one first, I'll then rebase this one.

Since releasing the decaffeinated code will be semver major (because of the bump to Node.js 6), it'd be great if you could checkout #16 as well.

[realityking]

@michalholasek Rebased!

[honzajavorek]

LGTM! Thank you!