Use require instead of global for browserify

src/renderers/pixi-renderer.js

@@ -80,6 +80,16 @@ Physics.renderer('pixi', function( parent ){
         // must be in node environment
         return {};
     }
+
+    // no pixi
+    if (!PIXI){
+        if (require){
+            // browserify, node-webkit, or requirejs
+            var PIXI = require('pixi.js');
+        }else{
+            return {};
+        }
+    }
 
     var Pi2 = Math.PI * 2
         ,colors = {