Fix Centos 6 build, repoforge/rpms#375

Author: jcheng5

docker/jenkins/Dockerfile.centos6.3

@@ -5,7 +5,7 @@ MAINTAINER Chad Barraford <chad@rstudio.com>
 RUN     rpm -Uvh http://dl.fedoraproject.org/pub/epel/6/x86_64/epel-release-6-8.noarch.rpm
 
 # RPMForge
-RUN     rpm -Uvh http://repoforge.mirror.constant.com/redhat/el6/en/x86_64/rpmforge/RPMS/rpmforge-release-0.5.3-1.el6.rf.x86_64.rpm
+RUN     rpm -Uvh http://mirror.chpc.utah.edu/pub/repoforge/redhat/el6/en/x86_64/rpmforge/RPMS/rpmforge-release-0.5.3-1.el6.rf.x86_64.rpm
 
 # SSH
 EXPOSE 22