I use this proguard file:
-dontskipnonpubliclibraryclasses -dontskipnonpubliclibraryclassmembers !code/simplification/arithmetic,!code/simplification/cas
It looks like the warning comes from the google library, have you tried something like that:
-dontwarn com.google.android.gms.** -keep class com.google.android.gms.**
I would also try without the dontwarn cause I would assume you want to be warned if there is something to be warned about!
dontwarn