Add instruction to install `hub` 2.x from Homebrew

Updated Homebrew recipe has been merged:
https://github.com/Homebrew/homebrew/commit/a746c26f8a7a6dd64850ba2cb9e76363bfca41e0
Merge pull request #653 from github/2_x_homebrew

Add instruction to install `hub` 2.x from Homebrew README