Spawning racker in rbenv context.

As prax isn't a gem you can't call rbenv exec on racker. The current implementation actually prevents racker form starting with a command not found error if you are using rbenv. rbenv relies on you having the gems installed for each ruby version and calls the bin file from it's shim directory.

This pull request in rbenv https://github.com/sstephenson/rbenv/pull/372 might allow us to change that without prax being a gem but seeing as prax is perhaps heading in the gem direction it might not matter?