Fix issue with java depends for Gentoo in autolibs

* Gentoo uses virtual/jre and virtual/jdk to fulfill JDK/JRE requirements
* These virtuals map to Sun, Oracle, and IcedTea (OpenJDK) implementations
* virtual/jdk for jruby-head virtual/jre for regular ruby
* Add ant-core to list of depends for jruby head
Merge pull request #1864 from cwgem/gentoo-jdk-jre

Fix issue with java depends for Gentoo in autolibs