Add src/ratpack to distZip task

When running the distZip Gradle task nothing in src/ratpack is included in the created zip file.

As I understand it only files in src/dist are included by default, this pull request explicitly includes src/ratpack.  I'm no Gradle expert so hopefully this is the best solution to the problem :)

See also the following Ratpack issue https://github.com/ratpack/ratpack/issues/180