Build: Fix regex that removes empty definitions

Bug
=====
The regex does not match the case ````define([]);```` , so it's not removed from the final output.
Reproduce the bug with this command:  ````grunt build:*````  (Building an empty jQuery)
The output should be an empty jQuery build, with only the wrappers ````"src/intro.js"````and ````src/outro.js"````
but it outputs also these two lines: 
````
      define([]); 
      define("jquery", function(){}); 
````
When loading this jquery.js build in the browser, it will give you a "define" is not defined error.
See bug ticket: http://bugs.jquery.com/ticket/15061#comment:2

Fix
=====
This PR fixes the first "define", I'm working to fix the other "define" that shouldn't be there neither.