Support ESLint 4

[prayerslayer]

ESLint is now available in version 4, here are the breaking changes for plugin developers.

I don't know if there is actually something to do here aside from bumping the peerDependency semver :)

[not-an-aardvark]

Actually, this already happened in #30, but it hasn't been released yet.

[not-an-aardvark]

Published to npm as 2.1.2.

[prayerslayer]

Cool, thanks for the super-fast response!