HQLPARSER-12 Migrating build to Gradle

Can be applied once https://github.com/hibernate/hibernate-jpql-parser/pull/11 is merged. The actual change for HQLPARSER-12 can be found [here](https://github.com/gunnarmorling/hibernate-jpql-parser/compare/HQLPARSER-10...HQLPARSER-12). ~~Note that I've left the Maven POM files as "safety net" for the time being, i.e. atm. the project can be built with both, Maven and Gradle.~~