specify RAILS_ENV when calling bundle assets-precompile (fixes #3638)

We currently disable the billing gem in production while it's on in development and test. Therefore bundler will not install its dependencies - in particular the braintree gem when deploying.
Since the RAILS_ENV was not specified rake was called with the default of 'development'. It therefore tried to load the development gems and failed when looking for 'braintree'. Specifying the production RAILS_ENV fixes this. It looks like we'll always need to specify RAILS_ENV when calling rake or we might want to export it to the environment in a separate task or the user config files such as .bashrc