[RFC] Travis: Prepare for  OS X builds.

Let's see what happens..
Because that's how a package would do it (see e.g. https://aur.archlinux.org/packages/ne/neovim-git/PKGBUILD). But that's probably unnecessarily complicated, so I changed it to just use the default (`/usr/local`) instead. Once neovim/deps#5 has the rebuilt dependencies again, I'll re-run this PR against these dependencies to make sure it works. I'll then revert to using `neovim/deps` and post a comment with the successful build.