Fixed brew install instructions in README.md and changed dependencies in brew

Addresses libuv issue in https://github.com/neovim/neovim/pull/99 
Changed README to point to correct homebrew installation instructions. https://github.com/neovim/neovim/issues/107
Edited neovim.rb to remove sha1sum dependency https://github.com/neovim/neovim/pull/70