Move filetype specific commands to ftplugin and ftdetect
Actually fix Command-T compilation with rbenv

Actually, this was partially a Mavericks issue. Mavericks built-in vim was compiled with 1.8.7,
whereas Mavericks ships with ruby 2.0.0. For this reason, we have to use an rbenv compiled ruby that matches
the version of ruby that came with vim.

This seems to work so far...