title: Unable to create call adapter in retrofit 2.0.0
body: <p>I am trying to integrate retrofit 2.0.0-beta in to my app.</p>
  
  <pre><code>{
       "success":true,
       "categoryDetails":[
      {
          "topicId":550,
          "categoryName":"Classshshs",
      “categoryImageUrl” : “http://s3-apdd-southeast-1.amazonaws.com/tv-dev/member/photo/883191-medium130.jpg”,
      },
      {
      "topicId":550661,
          "categoryName":"Cricket sss",
      “categoryImageUrl” : “http://s3-apdd-southeast-1.amazonaws.com/tv-dev/member/photo/883191-medium130.jpg”,
      }
  
     ],
     "totalCategoryCount":2
  }
  </code></pre>
  
  <p>My rest adapter</p>
  
  <pre><code>Retrofit retrofit = new Retrofit.Builder()
              .baseUrl("https://s3-ap-southeast-1.amazonaws.com").
                      addConverterFactory(GsonConverterFactory.create())
              .build();
  
      service=retrofit.create(RetrofitService.class);
  </code></pre>
  
  <p>My Call method</p>
  
  <pre><code>Call&lt;Object&gt; call=service.listRs();
      call.enqueue(new Callback&lt;Object&gt;() {
          @Override
          public void onResponse(Response&lt;Object&gt; response) {
              Log.e(TAG,"sucess"+response.toString());
  
  
  
          }
  
          @Override
          public void onFailure(Throwable t) {
  
          }
      });
  
  
  Caused by: java.lang.IllegalArgumentException: Unable to create call adapter for interface retrofit.Call
  for method RetrofitService.listRs
          at retrofit.Utils.methodError(Utils.java:201)
          at retrofit.MethodHandler.createCallAdapter(MethodHandler.java:51)
          at retrofit.MethodHandler.create(MethodHandler.java:29)
          at retrofit.Retrofit.loadMethodHandler(Retrofit.java:138)
          at retrofit.Retrofit$1.invoke(Retrofit.java:127)
          at java.lang.reflect.Proxy.invoke(Proxy.java:397)
          at $Proxy1.listRs(Unknown Source)
  </code></pre>