restrict visibility of Cask namespace in Formula

we had collision issues with homebrew's 'cask' formula (for the emacs
package manager project).

this keeps the in-project and in-formula visibility of the version
number via the old "namespaced global" strategy that keeps a `Cask`
constant out of view of the formula.
Merge pull request #2841 from phinze/version-namespace

restrict visibility of Cask namespace in Formula