Fix Macports build with openmpi

Problem was likely due to an upgrade of mpi in homebrew on travis

.travis.yml

@@ -110,6 +110,7 @@ install:
   - if [[ "$TRAVIS_OS_NAME" == "osx" ]] && test -z "$PLUMED_MACPORTS" ; then
       brew update > /dev/null ;
       brew install ccache ;
+      export PLUMED_MPIRUN="mpirun --oversubscribe" ;
     fi
   - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then sudo apt-get update -qq ; fi
 # install some package - these are fast, we install them anyway