Declare explicit dependency on s package

I believe this problem wasn't caught by the tests because the test
suite itself requires s, so s was already loaded during testing.

Fixes #130.

ido-completing-read+.el

@@ -6,7 +6,7 @@
 ;; Author: Ryan Thompson
 ;; Created: Sat Apr  4 13:41:20 2015 (-0700)
 ;; Version: 4.2
-;; Package-Requires: ((emacs "24.4") (cl-lib "0.5"))
+;; Package-Requires: ((emacs "24.4") (cl-lib "0.5") (s "0.1"))
 ;; URL: https://github.com/DarwinAwardWinner/ido-completing-read-plus
 ;; Keywords: ido, completion, convenience
 
@@ -86,6 +86,7 @@ not be updated until you restart Emacs.")
 (require 'ido)
 (require 'cl-lib)
 (require 'cus-edit)
+(require 's)
 
 ;;; Debug messages