Enable recursive submodule initialization for `heroku git:clone`

This is a pretty simple edit that allows convenient use of Git submodules from within the Heroku client. All it does is add the "--recursive" flag to the system call to Git; this will automatically initialize and update [potentially nested] submodules. This can be very useful for collaborating on large, complex projects! ([documentation](http://git-scm.com/docs/git-submodule))