Use --deployment in Travis

This is how we run in production, and we should reflect that in Travis
CI.
In addition, this configuration also helps catching two common problems:
- Developer forgetting to check in Gemfile.lock
- pull requests directly hacking on Gemfile.lock, whose chages got
  overridden later by a ``bundle install'' without deployment mode

Change-Id: I8d96fda84be2991e2f3f88e7c5226fca4492c4ef