Specify `sudo: false` to .travis.yml

For faster builds:
http://blog.travis-ci.com/2014-12-17-faster-builds-with-container-based-infrastructure/

Before:
`8 min 41 sec`
https://travis-ci.org/tricknotes/ember-rails/jobs/43919572

After:
`6 min 2 sec`
https://travis-ci.org/tricknotes/ember-rails/jobs/44481440