Fix broken Groovy compiler

The Groovy compiler shipped with Gradle 3.5 has a bug (https://issues.apache.org/jira/browse/GROOVY-8142) that can at times cause internal Gradle APIs to be referred to in the byte code even when the source code refers to public APIs only. This happened in the case of the Packer plugin, too. Some of these unintentionally referred-to internal APIs have been removed in Gradle 4.9, and thus the latest version of the Packer plugin produces an error when used with Gradle 4.9:

java.lang.IncompatibleClassChangeError: Class org.gradle.api.internal.project.DefaultProject_Decorated does not implement the requested interface org.gradle.api.internal.file.FileOperations

This PR upgrades the Groovy compiler used to build the plugin to fix the issue.

More information about the problem can be found here: https://github.com/gradle/gradle/issues/6027