Moved: use roundrect/tap

Swurl's cask now lives in RoundRect's single tap, roundrect/homebrew-tap, with every RoundRect app:

brew tap roundrect/tap
brew trust roundrect/tap
brew install --cask roundrect/tap/swurl

An install from this tap moves there on its own at the next brew update (tap_migrations.json). Then brew untap roundrect/swurl.