Update phinze/cask to caskroom/cask

It triggered this warning if brew cask had already been upgraded
to caskroom/cask and we ran rake:

> Warning: Could not tap phinze/homebrew-cask/brew-cask over
> caskroom/homebrew-cask/brew-cask
Don't copy the contents of an overwritten selection.
Document select-paste vim config.
Clarify the visual selection overwrite docs.
Merge pull request #161 from square/donovan/dont-copy-replaced-selections

Don't copy the contents of an overwritten selection.
Merge pull request #155 from slam/caskroom

Update phinze/cask to caskroom/cask
Clarify our position around adding new features to maximum-awesome.
Merge https://github.com/square/maximum-awesome