Mopub ads not showing
Solution 1:
It is ProGuard indeed. If you don't want it to "get rid" of Mopub, AdMob and MMedia code entirely, just put the following lines into proguard-project.txt file of your project:
-keep class com.google.ads.** {*;}
-keep class com.mopub.mobileads.** {*;}
-keep class com.millennialmedia.android.** {*;}
-keep class com.millennialmedia.google.** {*;}