capistrano-rbenv support added.

capistrano-rbenv has been added to gem dependencies and loaded in rvm plugin.

It is not necessary anymore to add it to your Capfile when using capistrano-activator
with rbenv based servers.

* capistrano-rbenv added to .gemspec
* capistrano/rbenv requirement added to rvm.rb plugin
* rvmsudo added to rvm_map_bins (used when exporting upstart with foreman)