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

@smarterclayton - please review
No, that's the upstream images responsibility 


> On Oct 29, 2014, at 3:50 PM, Michal Fojtik <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
> Maybe we should also do yum update to get latest updates when we rebuild this image.
> 
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