Update rakefile to support Centos

So this works but i'm not sure if it's how you guys want it to be setup.   I think one thing that would need to happen is to have you guys publish the centos images under your docker hub account.   Let me know if you have any thoughts on making this better.   
Actually one question @someword - what is the reason for adding the `-l` to the call to bash here? ("Make bash act as if it had been invoked as a login shell")