Adds capistrano/rvm support

`foreman:export` fails because foreman is not included in the `rvm_map_bins` array (much like the `bundle_bins` one). This adds a fix for it to ensure foreman will always load throught the rvm bin

PS: This is commited on top of my previous pull request (https://github.com/hyperoslo/capistrano-foreman/pull/26), so if for some reason that one is not accepted, jsut let me know and i'll revert the changes so it doesn't affect this one