Setup Travis Continuous integration

[leifdenby]

TODO

• get compilation working
  • installing dependencies: https://docs.travis-ci.com/user/installing-dependencies/
• check that model can run
• go through existing tests and decide whether they can be ported to run through pytest
  • existing tests: https://code.metoffice.gov.uk/trac/monc/wiki/MoncMain/MoncTesting
• make Adrian's bit-reproducability tests working

[leifdenby]

Future ideas:

• implement caching (as is done on https://github.com/d-meiser/mpi-travis) so that netcdf and mpich aren't downloaded and compiled every time, will decrease test run time
• further speed up testing by using the apt addon from travis instead of sudo apt-get (https://atchai.com/blog/faster-ci/)