Updated travis configuration to use pytest

[guyzmo]

changes are:

update to pytest
usage of latest pip version
usage of virtualenv
show coverage results as part of the log output

[ethantkoenig]

Why is this better than what we had previously?

[guyzmo]

1. pytest is a neat framework to work with, it's a rather minor change as is (it's built so it's compatible with unittest, but provide more abilities for debugging, like -x break at first failed test, --lf repeat last failure), and a bunch of fixtures that can be useful for testing
2. virtualenv + latest pip is to make sure that latest environment is running and is easily reproduceable
3. show coverage is just so that you don't need to push to travis to see changes in coverage 😉

[ethantkoenig]

@guyzmo Sorry for the delayed response; the last week has been busy.

I get why you might want to use pytest, virtualenv etc. locally, but it's not clear to me why our CI build needs to be updated.

[jag3773]

@guyzmo If you put these into a Makefile then that would make it easy to use locally.