refactor vim recipe
Disable command+t until "rvm use system" works
Simplify vim install; remove RVM dependency

This commit does a number of things to simplify and improve the vim
recipe:

- Re-enable Command-T.

- Remove dependency on RVM.  Both MacVim, and Command-T must be compiled
  with the system ruby.  Originally we used RVM to access the system
  ruby, but not everyone wants to use RVM.  The latest versions of
  homebrew use the system ruby automatically when installing MacVim, so
  that part is already fixed.  This commit compiles Command-T with the
  system ruby too, using similar tactics to homebrew.

- MacVim also installs a command line vim.  Use that one everywhere,
  instead of installing a separate command line vim.  This does NOT mean
  you have to use MacVim or that the command line doesn't work, only
  that they both rely on the same underlying install of vim.

- Remove dependency on mercurial, because we don't need to download vim
  from a mercurial repo.

- Stop blowing away ~/.vim on every run of soloist.  This was causing
  soloist to re-download every vim plugin on every run.  If you really
  want to force soloist to re-install vim, do something like the
  following:
    brew uninstall macvim
    rm -rf ~/.vim*
    rm -rf /Applications/MacVim.app

This commit builds on Doc Ritezel's initial work to modernize the vim
recipe.