Fixes #832

The `rescue` commands pertaining to `self.run` trigger errors when the `heroku-gem` is used rather than `heroku-toolbelt`.This is a small change that fixes the behaviour. Atleast until the time the installation of the `heroku` gem is completely stopped, I think we need to cover the edge cases. That said, I won't mind if this is rejected :)

This is my first PR to this repo and so, if I need to change anything, do let me know.