Check installed dependency min-version

This fixes #119 and #169

So to install the latest version of org-mode, you can use:

```elisp
;;; Cask
(package "example.el" "0.0.1" "example package")
(source "org" "http://orgmode.org/elpa/")
(depends-on "org" "8.2.7c")
```

Don't use package.el functions in cask.el directly, for compatibility reasons.