Install MacVim without depending on XCode

The brew install in [pivotal_workstation::vim](https://github.com/pivotal-sprout/sprout/blob/master/pivotal_workstation/recipes/vim.rb) depends on a full XCode
install. This recipe works with just the standalone XCode CLI tools.

Replacing pivotal_workstation::vim with this recipe should help us get closer to being able to use a Vagrant vm to test Sprout: https://www.pivotaltracker.com/story/show/55764518