Don't require a Rails backend

This is mainly intended for use with Passenger, for which no service
needs to be created or restarted.  To use, set `rails_deploy_backend` to
`False` or another falsy value.
Merge branch 'no-backend' of git://github.com/jscheid/ansible-rails_deploy into jscheid-no-backend
Merge branch 'jscheid-no-backend'