mpich2: devel/stable 3.1.1

Update to the latest release of MPICH (3.1.1)
It is the same right now, but this isn't always true. There just doesn't happen to be a beta or rc pending right now. It can be deleted, but it will come back in a few months. 

> On Jun 25, 2014, at 11:52 AM, Brett Koonce <notifications@github.com> wrote:
> 
> In Library/Formula/mpich2.rb:
> 
> > @@ -17,8 +17,8 @@ class Mpich2 < Formula
> >    end
> >  
> >    devel do
> > -    url 'http://www.mpich.org/static/downloads/3.1/mpich-3.1.tar.gz'
> If stable/devel is the same, should we drop this block?
> 
> —
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