move utility methods inside module namespace

Cask::Utils
detect need for "brew cleanup" in doctor

User will receive a warning if more than one copy of Homebrew-cask
is installed.

References: #4857
Merge pull request #4964 from rolandwalker/namespace_utils

move utility methods inside module namespace
Merge pull request #4963 from rolandwalker/doctor_brew_cleanup_needed

detect need for "brew cleanup" in doctor