use `Utils` namespace in 4 Casks

Per [CASK_LANGUAGE_REFERENCE.md](https://github.com/caskroom/homebrew-cask/blob/1ff7c99d9bd196f53c7994954248a848d8f57476/doc/CASK_LANGUAGE_REFERENCE.md#arbitrary-ruby-methods), best practice is to use a separate `Utils` namespace instead of defining any variables or methods at the top level of a Cask.

This keeps our own code from colliding with user code.  (We guarantee not to use the Utils namespace inside a Cask.)