Travis: switch to mpich2

- After moving to the xanadu version of FMS, Travis-CI was failing to link
  due to not being able to find mpi_comm_create_group(). Attempts to
  use the xenial distribution introduced different problems.
- This is a temporary work around.
  - We need to be able to build in newer distributions.

.travis.yml

@@ -4,14 +4,13 @@
 # This is a not a c-language project but we use the same environment.
 language: c
 dist: trusty
-sudo: false
 
 addons:
   apt:
     sources:
     - ubuntu-toolchain-r-test
     packages:
-    - tcsh pkg-config netcdf-bin libnetcdf-dev openmpi-bin libopenmpi-dev gfortran
+    - tcsh pkg-config netcdf-bin libnetcdf-dev mpich2 libmpich2-dev gfortran
 
 # For saving time...
 cache: