Decide upon minimum Node.js version

[Robbert]

Currently package.json states Node >=1.0 is supported, but that's a version never even tried. Perhaps a sane approach is to use the (latest) long-term support version and stick to that.

We might need to configure Wercker to use those Node.js versions for testing as well.

• [x] For now, use the first Node.js LTS release: >=4.2.0
• [x] Fix package.json