Make this plugin stop issuing fatal git warnings in non-git directories

If you use this plugin in a non-git directory, you get the following error message:

````
plugins ➤ heroku version                                                                                                                                                
fatal: Not a git repository (or any of the parent directories): .git
heroku-toolbelt/2.33.3 (universal-darwin12.0) ruby/1.8.7
````

This is because the plugin overrides git_remotes but doesn't carry out a check for the existences of a .git directory which the overridden method does.