Try `mpirun -np 2 --oversubscribe` to avoid an error on Travis

See https://github.com/envelope-project/laik/issues/155

"Beginning with version 3.0, OpenMPI's mpirun refuses to start more processes than there are physical processor cores on the machine."
1 job for topic/default/fix-travis in 8 minutes and 21 seconds
Status Job ID Name Coverage
  Test
passed #112025
tests

00:08:21