Fix elpa support for Emacs 24.4.

`package-desc-vers' is obsolete (replaced by `package-desc-version')
and `package-alist' has a different layout.

el-get-elpa also calls `el-get-rmdir' in some cases with
post-remove-fun as nil.

* methods/el-get-elpa.el (el-get-elpa-update-available-p): Use
  `package-desc-version' if available.
* el-get-core.el (el-get-rmdir): Only call post-remove-fun if non-nil.

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>
Minor fix: `sprintf' replaced by `format'.

(elisp is not C ;-)

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>
Merge pull request #1300 from ruediger/elpa24_4

Fix elpa support for Emacs 24.4