Fix Deprecation warnings

[swpalmer]

Running with Gradle 5.1.1 several warnings like the following appear when applying the plugin:

Unnecessarily replacing a task that does not exist has been deprecated. This is scheduled to be removed in Gradle 6.0. Try using create() or register() directly instead. You attempted to replace a task named 'jfxJar', but no task exists with that name already.
at build_40o5m1w9wlzcaq8iqfffzl9ll.run(C:\dev\MyApplication\build.gradle:17)
(Run with --stacktrace to get the full stack trace of this deprecation warning.)

[rehovicova]

Any progress on this one? Gradle 6 will not allow me to build my project now.