title: Unable to push to unqualified destination: master while deploying to demo branch
body: <p>I'm trying to create a demo branch for my application where suppliers can login to learn more about what it does. </p>
  
  <p>I have a master branch that deploys to production on Heroku. Additionally, I have a staging branch that deploys to my Heroku staging environment. In the Github App I created a branch off of the staging branch called 'demo'. </p>
  
  <p>When I deploy using CircleCI, I receive the following error:</p>
  
  <pre><code>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.
  
  git push git@heroku.com:foobar-demo.git $CIRCLE_SHA1:master returned exit code 1
  
  error: failed to push some refs to 'git@heroku.com:foobar-demo.git' Action failed: git push git@heroku.com:foobar-demo.git $CIRCLE_SHA1:master
  </code></pre>
  
  <p>My circle.yml looks like the following:</p>
  
  <pre><code>deployment:
    staging:
      branch: staging
      commands:
        - heroku maintenance:on --app foobar-staging
        #- heroku scale worker=0 --app foobar-staging
        - git push git@heroku.com:foobar-staging.git $CIRCLE_SHA1:master
        - heroku run rake db:migrate --app foobar-staging
        #- heroku scale worker=x --app foobar-staging
        - heroku maintenance:off --app foobar-staging
  
    demo:
      branch: demo
      commands:
        - heroku maintenance:on --app foobar-demo
        #- heroku scale worker=0 --app foobar-demo
        - git push git@heroku.com:lfoobar-demo.git $CIRCLE_SHA1:master
        - heroku run rake db:migrate --app foobar-demo
        #- heroku scale worker=x --app foobar-demo
        - heroku maintenance:off --app foobar-demo
  </code></pre>
  
  <p>The deployment flow to staging is working fine, the only issue is with my new demo branch. I tried deleting the branch and Heroku app (foobar-demo.herokuapp.com) and haven't had any luck. </p>