Update CentOS 6 Dockerfile
Update CentOS 6 Dockerfile

Replace the base image with an updated one that now contains
clang 3.9 with PGO support and also the new git. So we don't
need to build it here anymore.

livarcocc/cli-1,svick/cli,dasMulli/cli,livarcocc/cli-1,svick/cli,ravimeda/cli,livarcocc/cli-1,dasMulli/cli,dasMulli/cli,svick/cli,harshjain2/cli,johnbeisner/cli,johnbeisner/cli,harshjain2/cli,harshjain2/cli,ravimeda/cli,ravimeda/cli,johnbeisner/cli