Wip buck centos core

These changes were instigated as part of Gary bringing up issues he was seeing compiling on CentOS. It basically makes the JUnit tests for Java simpler (removes an unnecessary class) and changes configure.ac to loop through possible default Java locations (both Ubuntu and CentOS default installs are now detected). 

The test for "org.junit.rules.ExternalResource" is no longer needed as that class was removed. I've tested this code with JUnit 4.5 (which is the default in CentOS 6.3 and is 4 years old) and everything seems to work just fine. Ubuntu 12.04 has JUnit4 4.8.2, which also works just fine.