In Travis CI tests, set fast_finish flag to true

This change allows a build to be marked as failed as soon as a single job in
the build matrix has failed, providing faster feedback for devs.

See #5708

Props netweb

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8513 cdf35c40-ae34-48e0-9cc9-0c9da1808c22