Contribute Gradle plugin

Contributing the [Gradle plugin](https://github.com/ben-manes/gradle-jsonschema2pojo-plugin) required,
 - Updating to the latest version's `GenerationConfig` and
   package names
 - Adding to the Maven build. The Gradle plugin dependencies
   piggyback on Flyway's repository, as Gradle does not
   officially publish the dependencies for building plugins
   in another build tool.
 - Assigning copyright