update gradle to enable building with openjdk 14

[geodab]

Hello, I'm new to RxJavaFX and was trying to test it with a maven based project that depends on rxjava3. These are the changes I needed to successfully run ./gradlew publishToMavenLocal using openjdk 14. I also happen to be new to gradle but it looks like the more recent versions have helped simplify things a bit for handling java modules and scoping of dependencies. Hope this helps in the upcoming release with rxjava3 support!