CI: Use OpenMPI for mpi-linux-arm8 (#3433)

Author: evan-charmworks

.travis.yml

@@ -31,7 +31,8 @@ jobs:
     name: "MPI-Arm64"
     script:
     - sudo apt-get update
-    - sudo apt-get install -y gfortran mpich libmpich-dev
+    - sudo apt-get install -y gfortran libopenmpi-dev
+    - export OMPI_MCA_rmaps_base_oversubscribe=1
     - ./build all-test mpi-linux-arm8 -j3 --build-shared --with-production --enable-error-checking --enable-lbuserdata -g -Werror=vla || exit 1
     - make -C mpi-linux-arm8/tmp test
   - os: linux