Allow Zepto to work with jQuery in noConflict mode

* Check that window.$ is undefined rather than whether $ is a property
 of window.
* jQuery in noConflict mode sets window.$ to undefined which was causing
  Zepto to not be assigned

src/zepto.js

@@ -789,4 +789,4 @@ var Zepto = (function() {
 
 // If `$` is not yet defined, point it to `Zepto`
 window.Zepto = Zepto
-'$' in window || (window.$ = Zepto)
+if (window.$ == undefined) window.$ = Zepto