Fix CI on macOS due to GCC 8 breakage

 - There is a decent change that an additional commit updating the
   mpich bottles is required too...

Author: zbeekman

.travis.yml

@@ -15,7 +15,7 @@ env:
     - BUILD_TYPES="Debug RelWithDebInfo Release CodeCoverage"
   matrix:
     - GCC=6 OSX_PACKAGES="gcc@6 shellcheck" BUILD_TYPE="InstallScript"
-    - GCC="6 7" OSX_PACKAGES="gcc gcc@6 cmake shellcheck" BUILD_TYPE="cmake"
+    - GCC="6 7" OSX_PACKAGES="gcc@7 gcc@6 cmake shellcheck" BUILD_TYPE="cmake"
 
 matrix:
   fast_finish: true