These sometimes display useful RuboCop deprecations.

Currently the tests casks found in "/Homebrew/test/support/fixtures/cask/Casks" and "/Homebrew/test/support/fixtures/third-party/Casks" aren't included in the paths checked when running "brew cask style".

This change includes these test cask paths in the checked paths, and also includes the auto style fixes made by running `brew cask style --fix`.

This provides a more consistent version for `rubocop` than relying on
`Homebrew.install_gem_setup_path!` (and we really want `brew style` to
provide consistent output).

Fix the load path, requires and some filenames accordingly.

This requires updating to Rubocop 0.49.0 which will require some fixes
to rules, in Homebrew/brew and Homebrew/homebrew-core but opening this
for now so I remember.

This reverts commit 3e4547f5, reversing
changes made to 6edf9382.

Homebrew's actually ended up using a fair few gems. While we want to
avoid Bundler at runtime (and this PR still does that, in fact uses
Bundler even less at runtime than it did before) writing our own version
to use at build-time seems redundant.