Switch from Fedora docker base to centos, centos is a much smaller image

@smarterclayton - please review
I don't think you have to change here, just review my change in 304.  One problem I had - I was seeing the cross compile packages not get pulled but they didn't fail yum install.  Is there a way to -y without missing dependent packages?


> On Oct 31, 2014, at 10:33 AM, Adam Miller <notifications@github.com> wrote:
> 
> In hack/buildenv/Dockerfile:
> 
> >  MAINTAINER  Clayton Coleman <ccoleman@redhat.com>
> >  
> > -RUN yum install -y git hg golang golang-pkg-darwin golang-pkg-windows && yum clean all
> > +RUN yum -y install epel-release && yum clean all
> Yup, I don't know what I was thinking... after the && is a new instance of yum running so it will re-read the configs. I'm going to get more coffee and fix this. :)
> 
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