MPI version issues

The new version of the MPI code uses `mpi4py.futures` to create process pools. This module was added in mpi4py 2.1. 

For people at UC Davis - I need to figure out how to update mpi4py. In the meantime, you can keep using the version from the previous commit to run MPI jobs:
https://github.com/jdherman/ptreeopt/tree/73960f3828b8d057074465b1c39011cf5ab7530b