Cask uses the built-in emacs on OSX which does not support the --script argument which Cask requires. Is there some way to tell Cask to use the Emacs.app version?
I tried the solutions here: How to start "emacsformacosx" in terminal but they did not help.
I finally just entered the command supplied at the bottom of the cask file:
/Applications/Emacs.app/Contents/MacOS/Emacs -Q --script ~/.cask/cask-cli.el
This is not very elegant.