Allow branches.only specification in the form 'owner/repo@branch'

Not completely sold on the idea, since it is always possible to override this in a fork, but here is a proof-of-concept to address travis-ci/travis-ci#728.

Given:
```YAML
branches:
  only:
    - 'diaspora/diaspora@master'
    - 'diaspora/diaspora@develop'
    - 'mypod.example.org'
```

The build will happen if the request comes from the repo `diaspora/diaspora`'s `master` or `develop` branch, or the branch name is `mypod.example.org`.