Allow macvim to link against homebrew ruby

The comment in the old code that macvim only works with a framework build of ruby doesn't seem to be true any more.  This change adds the standard environment to pick up homebrew installed libraries, which is how the [vim](https://github.com/Homebrew/homebrew/blob/master/Library/Formula/vim.rb) formula works.
We aren't adding this to any more formulae. This should be modified to fix superenv instead. Also CC @jacknagel on this stuff.