Ending Java 6 Support

[Jean de Klerk]

Please note: since Java 6 extended support is being ended this December by Oracle, we will begin ending Java 6 support in early 2019, with release 1.28.0 as a tentative goal. Users may stay still use these libraries in Java 6 projects for some time, but going forward we will not ensure that these libraries work in such an environment. After 1.28.0, our supported versions will include Java 7 and onward. For Android users, we will continue our 4.0 support. For questions or concerns, please file an issue in the GitHub repository.