Use proguard on release builds

We don't need obfuscation (it's an open source project), but proguard
wil shrink the apk by stripping out unused code.

build.gradle

@@ -50,7 +50,7 @@ android {
 	}
 	buildTypes {
 		release {
-			runProguard false
+			runProguard true
 			proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.txt'
 		}
 	}

proguard-rules.txt

@@ -16,6 +16,8 @@
 #   public *;
 #}
 
+-dontwarn javax.naming.**
+
 -keep class * extends java.util.ListResourceBundle {
     protected Object[][] getContents();
 }