Fix MacVim Yosemite preference

There are two MacVim variants: one compiled for Mountain Lion and below, and another for Mavericks and above.

[macvim.rb](https://github.com/caskroom/homebrew-cask/blob/master/Casks/macvim.rb) cask downloads version for Mountain Lion when on Yosemite, which results in MacVim application failing to open. This patch fixes the problem.