Build using Docker

This should result in quicker builds, at least things like quicker build startup, faster builds, more RAM and CPU:

http://blog.travis-ci.com/2014-12-17-faster-builds-with-container-based-infrastructure/

Enabled with `sudo: false`.

The main test bit is network dependent, so it probably won't affect that. But might as well get all the benefits available.

I also added verbose for the tests so you can see which pass/fail, and print the test log on failures. This didn't actually trigger for the timeouts, but may be useful in the future.