Update MPICH bottle due to formula revision bump

 - GCC 7.1 was released and added to Homebrew
 - Because of this formula revision bumps are being mode for formulae
   with GCC dependencies etc.
 - As a consequence, our custom bottle will no longer install due to the
   revision bump, despite the fact that we are using gcc@6 still for
   MacOS testing on Travis-CI
 - I didn't try just renaming the bottle `s/3.2_2/3.2_3/` which might
   have been easier and faster than rebuilding it... oh well

Author: zbeekman

.travis.yml

@@ -7,8 +7,8 @@ env:
     - MPICH_URL_HEAD="http://www.mpich.org/static/downloads/$MPICH_VER"
     - MPICH_URL_TAIL="mpich-${MPICH_VER}.tar.gz"
     - MPICH_DIR="$HOME/.local/usr/mpich"
-    - MPICH_BOT_URL_HEAD="https://github.com/sourceryinstitute/opencoarrays/files/452136/"
-    - MPICH_BOT_URL_TAIL="mpich-3.2_2.yosemite.bottle.1.tar.gz"
+    - MPICH_BOT_URL_HEAD="https://github.com/sourceryinstitute/OpenCoarrays/files/976766/"
+    - MPICH_BOT_URL_TAIL="mpich-3.2_3.yosemite.bottle.1.tar.gz"
     - FC=gfortran-6
     - CC=gcc-6
     - CXX=g++-6