Update Gruntfile.js fixing undefined jQuery check

The check to see if jQuery was defined as incorrect causing the "Bootstrap requires jQuery" message to only be displayed if jQuery is defined and has a value which equates to `false`.

Gruntfile.js

@@ -15,7 +15,7 @@ module.exports = function(grunt) {
               '*\n' +
               '* Designed and built with all the love in the world by @mdo and @fat.\n' +
               '*/\n',
-    jqueryCheck: 'if (!jQuery) { throw new Error(\"Bootstrap requires jQuery\") }\n\n',
+    jqueryCheck: 'if (typeof jQuery === "undefined") { throw new Error(\"Bootstrap requires jQuery\") }\n\n',
 
     // Task configuration.
     clean: {