Allow specifying of branch name or SHA1 ref (defaults to master)

Adds a field allowing specification of a branch name or SHA1 ref to the index view.

Supports the following URLs:

* `/user/repo` (existing URL format, validates `.travis.yml` on `master` branch)
* `/user/repo/` (same as above)
* `/user/repo/ref` (validates `.travis.yml` on `ref` branch/commit)