Support new EcmaScript syntax

[ericandrewlewis]

Can we update the esprima dependency to the latest stable ~4.0.0?

I've tested locally based on the 0.1.2 release (commit d615e5c) and all tests pass with the updated dependency.

[k4b7]

@ericandrewlewis this project is not being actively developed. If you're using it for a project it's probably best to update that dependency in your fork.