CrashlyticsMissingDependencyException when integration with MarmaladeSDK and Firebase

Question

I tried to use Crashlytics instead of FirebaseCrash Reporting. I added all relevant Libraries: crashlytics, fabric, answer, beta and everything compiled and started properly on my Android device. I'm working with MarmaladeSDK which has no AndroidStudio/gradle integration. I need to add every lib by myself to the build system (jars, Android Manifest, Resources mainly).

I have two issues:

1. Crashlytics is not started automatically. I had to call Fabric.with(this, new Crashlytics()); Which aar library is reponsible for starting the crashlytics services?

2. I got an CrashlyticsMissingDependencyException with the error message:

   This app relies on Crashlytics. Please sign up for access at 
   https://fabric.io/sign_up, install an Android build tool and ask a team 
   member to invite you to this app's organization.
   

How does chrashlytics gets the knowledge that Firebase is the new destination and that there is no need to register at the fabric.io website?

I have no idea what apply plugin:'io.fabric' does? Can I mimic this behaviour even without Android studio? My main issue is probably that gradle does some magic which I don't understand and I assume that apply plugin:'io.fabric' could be responsible for that.