How to use google play service 31 to admob unity 4.6.9

Hi every one !

I'm trying to add google admob to my project. I'm using google admob plugin. But I can't find google play service lib with my google play service revision 31.

I'm finding solution and following Athalos solution. But I have error:

 AndroidJavaException: java.lang.ClassNotFoundException: com.google.android.gms.ads.AdSize
                                           at UnityEngine.AndroidJNISafe.CheckException () [0x00000] in <filename unknown>:0 
                                           at UnityEngine.AndroidJNISafe.CallStaticObjectMethod (IntPtr clazz, IntPtr methodID, UnityEngine.jvalue[] args) [0x00000] in <filename unknown>:0 
                                           at UnityEngine.AndroidJavaObject._CallStatic[AndroidJavaObject] (System.String methodName, System.Object[] args) [0x00000] in <filename unknown>:0 
                                           at UnityEngine.AndroidJavaObject.CallStatic[AndroidJavaObject] (System.String methodName, System.Object[] args) [0x00000] in <filename unknown>:0 
                                           at UnityEngine.AndroidJavaObject.FindClass (System.String name) [0x00000] in <filename unknown>:0 
                                           at UnityEngine.AndroidJavaClass._AndroidJavaClass (System.String className) [0x00000] in <filename unknown>:0 
                                           at UnityEngine.AndroidJavaClass..ctor (System.String className) [0x00000] in <filename unknown>:0 
                                           at GoogleMobileAds.Android.Utils.GetAdSizeJavaObject (GoogleMobileAds.Api.AdSize ad

I'm researching my error and google play service of my project can't be found.

So what can I do to add or find google play service lib with my google play service revision 31.

Pls help me :(