Fix #207, drop support for Node versions <4.0.0

[nifgraup]

Required to get the tests green in #209.

Alternatively, version 4 could be the minimum version to build terser, but test runs could still execute on older versions of node.js.

[fabiosantoscode]

I've added iojs and 4 to the .travis.yml. They should work, but you need to specify more process.versions to skip jetstream on.

Also, can you change our minimal supported version in the readme? Or add a note of it?

[fabiosantoscode]

Nevermind, I can add that note to the README. Will merge and release when #204 is fixed to avoid confusion.

[nifgraup]

Jetstream is something I haven't familiarized myself with.

I will let you add to the README and decide if you want to raise just the build requirements to >= 4.0.0 or also the runtime requirements.

I did raise the minimum "engines" in package.json to node >= 4.0.0, I'm not sure if that refers to just to runtime requirements or also build reqs.

[fabiosantoscode]

@nifgraup "engines" is OK as you left it.

I'll merge this PR. Thanks for doing god's work.