Decouple Gradle plugin releases from Graal releases

[sschuberth]

          Hi @sschuberth, thank you for the report. Unfortunately, our release engineering is deeply tied into Graal's and we're not able to release the plugin outside of the normal release schedule. The next release should happen on 18th of March.

@steve-s could you please check if the bug is really fixed?

Originally posted by @msimacek in #462 (comment)

[sschuberth]

To my understanding there isn't really a technical restriction why Grable plugin and Graal releases must be synced. As Graal releases don't come too often, and the plugin might have received fix meanwhile, it would be great to be able to release the plugin independently of new Graal versions.