Fixes for heroku-push on Windows

* fix for https://github.com/DjangoGirls/tutorial/issues/143

* `heroku-push` requires existing git repo to use .gitignore (Proof: https://github.com/ddollar/heroku-push/blob/master/lib/push/heroku/command/push.rb#L82), without it it will just push everything in current directory.
Without git repo `.gitignore` won't work