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
use _system_version instead of /etc/centos-release, fix #1866
Merge branch 'master' of github.com:wayneeseguin/rvm