disable RSpec test which writes outside test directories

invoking `Cask.init` attempts to actually `mkdir /custom/caskdir`

Per https://github.com/caskroom/homebrew-cask/blob/2db68b6bfa7c380db4c9ac5883bafc408c561195/lib/cask.rb#L57

which is really a bug in `Cask.init`.

cc @radeksimko