fix(loader): allow AMD loader to be used within Node env

Because of the ordering of the loader, it's not possible use AMD within a Node environment.

src/loader.js

@@ -1,16 +1,16 @@
 var root = this;
 
-// CommonJS/nodeJS Loader
-if (typeof module !== 'undefined' && module.exports) {
-  module.exports = showdown;
-
 // AMD Loader
-} else if (typeof define === 'function' && define.amd) {
+if (typeof define === 'function' && define.amd) {
   define(function () {
     'use strict';
     return showdown;
   });
 
+// CommonJS/nodeJS Loader
+} else if (typeof module !== 'undefined' && module.exports) {
+  module.exports = showdown;
+
 // Regular Browser loader
 } else {
   root.showdown = showdown;