Add facility to build custom ruby-2.0.0 RPMS and install them.

This pull-request replaces in the centos.lib.sh script the installation of ruby-2.0.0 vi rvm
with a custom-built ruby-2.0.0 RPM that will replace the native Centos 6.5 ruby-1.8.7 RPM 
that is temporarily installed so that we can build ruby-2.0.0 from the latest ruby GIT repo for 2.0.0.