Support older versions of Gradle

[DPUkyle]

The Groovy and Scala plugins, upon which this is based, leverage some non-public Gradle APIs which change fairly frequently in Gradle point releases.

As part of integrating this plugin with Guidewire's Gradle build (which is based on 2.3 rc3), we've uncovered quite a few CNF exceptions.

This issue is a placeholder to test plugin execution against older Gradle releases and verify compatibility.