Allow deploying from a detached head

I'm having an issue deploying to brand new heroku servers.
The first push to a new server is failing with:
```
error: unable to push to unqualified destination: master
The destination refspec neither matches an existing ref on the remote nor
begins with refs/, and we are unable to guess a prefix based on the source ref.
error: failed to push some refs to 'git@heroku.com:blue-register-18.git'
```
This is important to me as I'm having travis-ci create a new heroku server against each pull request.
Seems to be the same issue discussed here: http://git.661346.n2.nabble.com/error-with-git-push-origin-HEAD-newbranch-td6333596.html
As far as I can tell, the "master" branch is not yet a known branch on the new heroku server, and the local HEAD is detached so git doesn't know whether the "master" it's pushing is a branch or tag. This is resolved by specifying that it is, indeed, a branch: refs/heads/master.
This may be git-version dependent, as mentioned in the mailing list. I experience the error through travis-ci but have been unable to reproduce the error locally locally (git version 1.7.5.4).

Thanks!