Remove execute bit from bin/brew-cask.rb

bin/brew-cask.rb is meant to be required by Homebrew, not actually executed by
the shell. The execute bit is causing rbenv-binstubs to pick up
bin/brew-cask.rb is an executable and is shimming it. Which then causes
Homebrew to attempt to `require` the rbenv-shimmed shell script rather than the
ruby script.

IMO, the *correct* behavior is for non-executable ruby scripts to never be
placed in bin/ in the first place. But as Homebrew requires this behavior for
its external commands, removing the execute bit is a workaround.