[travis-ci] Allow fast finishing

With a single test run easily exceeding half an hour of cumulative time, we should be getting build results a bit sooner. This PR allows [fast finishing](http://docs.travis-ci.com/user/build-configuration/#Fast-finishing) in Travis CI. As a result, a build is being marked as "passed" if either:

1. A job that is not allowed to fail has indeed failed
2. all remaining jobs are allowed to fail.

It is important to note that this doesn't actually *skip* any jobs (cf travis-ci/travis-ci#2260).

See also: http://blog.travis-ci.com/2013-11-27-fast-finishing-builds/