Make Heroku destination refspec more specific.

Pushing to a new Heroku application (not pushed to yet) was failing with the below output.  Making the refspec more specific allowed the push to go through.

```bash
$ git push heroku HEAD:master --force
Initializing repository, done.
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:application-name.git'

```