To enable ProGuard you must set useproguard variable under android section to true:
android:
useproguard: true
If you have non standard rules in your application, that uses such features as Reflection API it will be beter for you to make your own rules config. To include it and mearge with our base config you should write path to your config in variable proguardrules:
android:
proguardrules: "rules.pro"