Since Crashlytics is now part of Fabric the initialization process has changed, but is still simple. Instead of using Crashlytics.start() you should now use, but in the Application creation:
public class App extends Application {
    ...
    @Override
    public void onCreate() {
        super.onCreate();
        Fabric.with(this, new Crashlytics());
    }
    ...
}
For a more richer example, see how Cannonball canonical sample app is doing:
public class App extends Application {
    ...
    private TwitterAuthConfig authConfig;
    ...
    @Override
    public void onCreate() {
        super.onCreate();
        authConfig = new TwitterAuthConfig(BuildConfig.CONSUMER_KEY, BuildConfig.CONSUMER_SECRET);
        Fabric.with(this, new Crashlytics(), new Twitter(authConfig), new MoPub());
    }
    ...
}
This code is available at: https://github.com/twitterdev/cannonball-android/blob/master/app/src/main/java/io/fabric/samples/cannonball/App.java#L96-L98