Auto-install cask-mode when needed

core/prelude-packages.el

@@ -136,6 +136,7 @@ PACKAGE is installed only if not already present.  The file is opened in MODE."
     ("\\.coffee\\'" coffee-mode coffee-mode)
     ("\\.css\\'" css-mode css-mode)
     ("\\.csv\\'" csv-mode csv-mode)
+    ("Cask" cask-mode cask-mode)
     ("\\.d\\'" d-mode d-mode)
     ("\\.dart\\'" dart-mode dart-mode)
     ("\\.elm\\'" elm-mode elm-mode)