.exports: Remove `export HOMEBREW_CASK_OPTS`

It’s not needed anymore now that the `Caskfile` has been removed.

Closes #446.
Merge pull request #1 from mathiasbynens/master

.exports: Remove `export HOMEBREW_CASK_OPTS`
Merge pull request #2 from flexmonkeypatch/master

merge master into airK?