emacs cask: install to site-lisp

Allows the following in `init.el`:

```lisp
(require 'cask)
(cask-initialize)
```

Otherwise a brittle path into `Cellar/` is necessary:

```lisp
(require 'cask "/opt/boxen/homebrew/Cellar/cask/0.6.0/cask.el")
(cask-initialize)
```