Add tpope/vim-rvm to the plugin list

tpope/bundler won't work without this plugin on a project
that uses dvm when mvim is not launched from within the
project's directory.
Using the command :Rvm will source the right GEM_HOME
so that :Bedit [gem] is able to look in the right place.
Add support for sourcing the right rvm env on bufenter

This extra configuration plugin calls :Rvm on every BufEnter
making sure that the right GEM_HOME and GEM_PATH are
set.