automatically transition to new Tap name/location

At every command execution
- invoke `rename_taps_dir_if_necessary` to help with Homebrew's Tap migration.
  (Homebrew's migration seems to sometimes fail to fire, see #4096)
- untap `phinze/cask`, and tap `caskroom/cask`, to help with our own move
  to a GitHub organization
After a few months, this can be reverted.
Merge pull request #4169 from rolandwalker/implicit_tap_transition

automatically transition to new Tap name/location