rename hockeyapp.rb to hockey.rb

to conform with naming conventions.

It seems we have visited the naming here before in #4286, but not covered [Remove the term "app" from the end, if the developer styles the name like "Software App.app"](https://github.com/caskroom/homebrew-cask/blob/163e52aa853c623345bf2398b3e0ea5bce0dff4e/doc/CASK_NAMING_REFERENCE.md#canonical-names-of-apps).