Tweak travis file to fix failing tests

There are 4 test which fail, only on Travis CI, on all existing branches - develop, master, etc. They pass when the tests are run locally. Two of the four failures are fixed by this tweak to the travis.yml file.

This follows on from the final comments at https://github.com/CakeDC/migrations/pull/148#issuecomment-35208432

I'm unable to figure out what is causing the failure in the other two tests, but you can see the failures at https://travis-ci.org/joshuapaling/migrations/jobs/19028060