[ticket/12472] Set fast_finish for travis

Now, a build will finish as soon as a job has failed,
or when the only jobs left allow failures.
http://docs.travis-ci.com/user/build-configuration/#Fast-finishing

PHPBB3-12472
Merge remote-tracking branch 'github-nickvergessen/ticket/12472' into develop-olympus

* github-nickvergessen/ticket/12472:
  [ticket/12472] Set fast_finish for travis