Hardened security measures ("notarization") promised by Apple hit the market in the meantime and we are lagging in following this. We are working on the issue and new version will soon comply with these new measures.
In case of Marvin you may still run it using command line. Open Terminal and type:
Open System Preferences/Security & Privacy where you see the app under the Allow apps download from section. Press Open Anyway.
Next run from command line may bring up dialogs asking for your permission. Allow them to proceed with it.