drop Node.js v4 support

[boneskull]

Per #1599 this looks like something you want to be doing.

In a little under two weeks (April 30, 2018), Node.js v4 will be End-of-Life'd.

I wanted to encourage the Ava team to drop support for this version as soon as is feasible. Mocha will also be dropping support ASAP.

Thanks!

[harrysarson]

@boneskull they are one step ahead of you it seems: #1760

[boneskull]

great—was just going off of latest published version.

[novemberborn]

Hey @boneskull, thanks for dropping by!

We're currently shipping beta releases that use Babel 7, which is itself in beta. So unfortunately the latest package on npm is looking a little outdated. But yea, farewell Node.js 4 🎈