Add gtk-mac-integration to gnome_devel_whitelist

Fix tap unavailable error for migrated Cask taps.

Fix ignore file

list.rb: use CLI::Parser and improve handling of flags

Fix comparison with Version::NULL on RHS

brew: Conciser error when run from nonexistent directory

Refactoring

Silence `Parser` compatibility warning.

Fall back to `CaskLoader.load` to get a better error message.

This was accounted for in Version::NULL.<=>, but not in Version#<=>,
so these could only be compared if Version::NULL was the left hand
side.

The check had to go above the check that `other` is a version because
`Version::NULL`'s anonymous class does not inherit from `Version`.
(The type check feels like it's probably a smell, but out of scope).

Fix `rspec` deprecations.

Update Cask tap regex.

Update Homebrew-Cask references.

Reset `repo_var` so it actually is re-computed.

This reverts commit 4475651b.

Activate Homebrew-Cask tap migration.

* --multiple now assumes --versions
* Use CLI::Parser (that, in turns, uses OpenStruct) to handle arguments

cli_parser: Add depends_on, required_for and conflicts for switch options