Make the Homebrew/cask and Homebrew/homebrew-core style more closely
match the rest of Homebrew.
To accomplish this:
- Run `brew cask style` to ensure we don't break style there when
making changes or upgrading RuboCop in Homebrew/brew.
- Fix the HomepageMatchesUrl cop to better handle weird input.
- Remove the now unneeded `.rubocop_shared.yml`
- Fix the cask fixtures with `brew cask style --fix`.
- Share more style between Homebrew/brew, casks and formulae.
Anywhere we can use `blob/master` we can use `blob/HEAD` instead. This
will make life easier if we ever rename our default branch in future
(once/if Git and GitHub provides the necessary tooling to do so).
