Git doesn't seem to notice when I change the file extension from all
caps to lowercase, yet browsers can't seem to find the file when the
case is wrong. I'm going to remove it, commit, then add it with
lowercase and commit.

`make html` now does a clean before building html docs.

This will hopefully allow me to show images on the pypi site and in
GitHub.

1) typo likely caused 'command not found'
2) codecov needs to be in 'install' so that I can use coverage when I
`coverage run setup.py test`
3) It still makes sense to keep coverage in the test requires, I think,
so  you can use it locally after you install.

See issue travis-ci/travis-ci#2650 for the suggestion to update pip and
request the wheels for scipy and numpy.

travis-ci/travis-ci#2650 (comment)

I don't use this file in my distribution, but it includes things like
Sphinx, which I use. This file could be useful for someone else who
wants to build the sphinx docs.