travis fast-finish builds

For a while, Travis CI has supported allowed failures in your build
matrix - jobs that are allowed to fail, without affecting the status of
the entire build.

- http://blog.travis-ci.com/2013-11-27-fast-finishing-builds/
-
http://docs.travis-ci.com/user/build-configuration/#Rows-That-are-Allowe
d-To-Fail