Add casks to deal with test dependencies

Following this [PR](https://github.com/haskell/haskell-mode/pull/370), I
needed to add some unit test lib to tests things.

As no dependencies seems necessary until now, I just added a temporary HACK
https://github.com/haskell/haskell-mode/pull/370#discussion-diff-20121622.

Cask [seemed a proper solution for
this](https://github.com/haskell/haskell-mode/pull/370#discussion_r20136791).

Here is a way to make Cask run only the tests for now.

The only problem now seems that in ci-travis, the emacs-snapshot is an
emacs 23.3 and Cask only supports emacs >= 24.

Cheers,