Improved continuous integration

[ASvyatkovskiy]

This PR implements a set of changes to the .travis.yaml and cleans up requirements/setup files. The main goal is to make mpi4py a part of the setup.py script:

try:
    os.environ['MPICC'] = subprocess.check_output("which mpicc", shell=True).decode("utf-8") 
except:
    print ("Please set up the OpenMPI environment")
    exit(1)

which requires OpenMPI installed prior to that. Adding install-mpi.sh and adjusting the travis matrix does that:

matrix:
  include:
    - env: MPI_LIBRARY=openmpi MPI_LIBRARY_VERSION=2.0.0
      python: 2.7
    - env: MPI_LIBRARY=openmpi MPI_LIBRARY_VERSION=2.0.0
      python: 3.6

Finally, README.md is updated.