After update - Error:Failed to find: com.google.android.gms:play-services:5.2.8

try using com.google.android.gms:play-services:5.2.08 instead of 5.2.8.

you can also look for the right version number in your services directory:

.. /android-studio/sdk/extras/google/m2repository/com/google/android/gms/play-services/

Check your sdk extras packages, you need to install:

• Google Play services
• Google Repository

For me, the thing was that I didn't have the second one installed and then "---:5+" worked as described here:

https://developers.google.com/mobile-ads-sdk/docs/admob/android/quick-start