Updated UMD definition

As per this PR: umdjs/umd#22

Author: jacobawenger

build/header

@@ -10,14 +10,14 @@
 
 ;(function (root, factory) {
   "use strict";
-  if (typeof exports === "object") {
-    // CommonJS
-    module.exports = factory();
-  } else if (typeof define === "function" && define.amd) {
+  if (typeof define === "function" && define.amd) {
     // AMD
     define([], function() {
       return (root.ReactFireMixin = factory());
     });
+  } else if (typeof exports === "object") {
+    // CommonJS
+    module.exports = factory();
   } else {
     // Global variables
     root.ReactFireMixin = factory();