modernize vim recipe; remove rvm dependency

These commits modernize and improve the vim recipe.  See the commit messages for more details, but the basics are:

* Stop taking the "if the recipe fails once, give up" approach.  Instead, make it easier to run the recipe repeatedly.  It isn't idempotent (it could update macvim or your vimconfig) but those are side-effects of every recipe that uses homebrew or git :sync.
* Remove dependency on RVM, but still reliably compile Command-T
* Use MacVim's command line version of vim as the system vim.  Don't download and compile other vims.