Use :command_prefix to ensure that rvm will always be initialized if the RVM recipe is used

I had this patch for some time now, but I needed a couple of patches to make their way into vlad and rake-remote_task upstreams.

Currently, even if we use the vlad-rvm plugin, nothing ensure that we will load RVM for each of our command, this is required for commands such as precompiling assets, bundle install and rails migrations.