Upgrade Gradle build tools to version 4.0.0

Things to observe:

requires using (at least) version 6.1.1 of the Gradle wrapper

the findbugs plugin ceases to exist in 4.0.0

breaks the publishing of snapshots

This update could potentially fix the issue we have with runtime crashes due to the addition of the Firebase Cloud Messaging dependency when using the Gradle build tools version 3.6.1 (it is worth noting that there aren't crashes when downgrading to the Gradle build tools version 3.5.3)