Travis shouldnt test gh-pages and friends

This adds a blacklist of branches that Travis shouldn't run tests against.

## Verification

- [x] Builds like [this one](https://travis-ci.org/rapid7/metasploit-framework/builds/42902010) should cease to be.

gh-pages and metakitty are the builds and the source (respectively) for http://rapid7.github.io/metasploit-framework, and shouldn't be tested yet.