Travis: Node.js runtime instead of V8

Travis fails on bundle install when installing therubyracer gem. This
means we cannot use V8 as JavaScript runtime on Travis.

So:

* Use Node.js on Travis, which should be pre-installed on workers.

* Keep the option of using therubyracer/V8 on development machines
  (because Node.js is not packaged e.g. for Fedora right now).
Travis: assets precompilation test

Test that assets can be precompiled for production use.