Update Centos 6.4 guide to obtain git via RPM instead of compilation

Recently I installed Gitlab 6.2 on a CentOS6.4 system using the excellent "GitLab recipe" that has been put together.  I knew I could get a modern git RPM package from a trusted repository instead of compiling it, so I've updated the instructions which simplifies things a bit.  Everything worked out well with my GitLab install using the alternate git installation route.

Sidenote: I'm working on some Ansible scripts to auto-magically deploy GitLab at https://github.com/psftw/simple-ansible but it is not ready for prime time yet.