fix(index): detect System or require api and load appropriately

This fixes #110

src/index.js

@@ -51,11 +51,19 @@ function ready(global) {
 
 function ensureLoader(){
   if(!window.AureliaLoader){
-    return System.normalize('aurelia-bootstrapper').then(function(bootstrapperName){
-      return System.normalize('aurelia-loader-default', bootstrapperName).then(function(loaderName){
-        return System.import(loaderName);
-      })
-    });
+    if(window.System){
+      return System.normalize('aurelia-bootstrapper').then(bootstrapperName => {
+        return System.normalize('aurelia-loader-default', bootstrapperName).then(loaderName => {
+          return System.import(loaderName);
+        })
+      });
+    } else if(window.require){
+      return new Promise((resolve, reject) => {
+        require(['aurelia-loader-default'], resolve, reject);
+      });
+    }else{
+      throw new Error('No window.AureliaLoader is defined and there is neither a System API (ES6) or a Require API (AMD) available to load your app.')
+    }
   }
 
   return Promise.resolve();