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

@smarterclayton - please review
That explains what I was seeing in #304.  If that's the case don't bother changing here, I'll patch in 304.  You should take a look there as well for the other changes with regards to release behavior in the near term.


> On Oct 31, 2014, at 10:06 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
> I actually don't think it will work because epel-release lays down a yum repo file so the following run needs to re-read the configs and pick up the new repo. I'll test and verify, but that is why I originally did this separately.
> 
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