generate pom from gem in case we have gem with java platform

generate pom from gem in case we have gem with java platform so the jar dependencies are translated as dependency inside the pom.

fixes https://github.com/torquebox/rubygems-servlets/issues/7

this is a really important feature for gem with jar dependencies like(see requirements at the end): https://rubygems.org/gems/hermann/versions/0.18.1-java