Remove OS X requirement

As linuxbrew [1] is quite popular [2] now

[1]: https://github.com/Homebrew/linuxbrew
[2]: https://github.com/neovim/neovim/wiki/Installing
Agree on `(darwin|linux)*`. I'm going to rebase the pull request now.

@PaBLoX-CL, at first I thought it's a port, too. But actually they share some codebase and can be merged like this https://github.com/Homebrew/linuxbrew/commit/4c8e2b0a6f969df1c0ebdaf3be2e3d12ca6bb0eb