Fix ubuntu GCC version diff & use brewed mpich

Author: zbeekman

.travis.yml

@@ -24,7 +24,7 @@ matrix:
       env:
         - BUILD_TYPE="InstallScript"
           GCC=8
-          OSX_PACKAGES="gcc shellcheck cmake"
+          OSX_PACKAGES="gcc shellcheck cmake mpich"
           # cmake should be removed, but we should be installing binaries
           # because building from source takes forever
     -
@@ -68,7 +68,7 @@ matrix:
       <<: *ubuntu
       env:
         - BUILD_TYPE="InstallScript"
-          GCC=6
+          GCC=7
       addons:
         apt:
           sources: *linuxsrcs