Ensure package-pinned-packages is bound before referencing it.

Add a bound-and-true-p guard to package-pinned-packages before
referencing it in use-package-ensure-elpa.

Package pinning was introduced in Emacs 24.4, and hence
package-pinned-packages in unbound by default in earlier versions.

Relevant commits:
	 83aec5a Merge pull request #367 from ketbra/master
	 ec22352 Make pin and ensure compatible

Fixes #375

use-package.el

@@ -500,7 +500,8 @@ manually updated package."
 (defun use-package-ensure-elpa (package &optional no-refresh)
   (if (package-installed-p package)
       t
-    (if (and (not no-refresh) (assoc package package-pinned-packages))
+    (if (and (not no-refresh)
+             (assoc package (bound-and-true-p package-pinned-packages)))
         (package-read-all-archive-contents))
     (if (or (assoc package package-archive-contents) no-refresh)
         (package-install package)