The string -dontwarn com.squareup.okhttp.OkHttpClient.*does not correlate with Warning: com.squareup.picasso.OkHttpDownloader.
You might try to change the package name in proguard.cfg
Added: Try adding to proguard.cfg
-dontwarn
-injars bin/classes
-injars libs
-outjars bin/classes-processed.jar
-optimizationpasses 5
-dontskipnonpubliclibraryclasses
-dontskipnonpubliclibraryclassmembers
-dump class_files.txt
-printseeds seeds.txt
-printusage unused.txt
-printmapping mapping.txt
-optimizations !code/simplification/arithmetic,!field/*,!class/merging/*
-allowaccessmodification
-repackageclasses ''
-keepattributes Signature
I understand the file proguard.cfg or proguard-project.txt (proguard-android.txt). Try this code in both files.