Clean up coverage instructions

[brettcannon]

Thanks to Codecov and Travis, not only do we have continuous code coverage reports, but we have included instructions on how to run coverage.py as well. Once python/core-workflow#18 is dealt with we should strip those instructions down to their bare minimum and rely more on Codecov for where people can get current coverage results and Travis for instructions on how to run things.

[brettcannon]

Another thing we may want to do is emphasize people should focus on low-hanging fruit when it comes to coverage instead of trying to get every hard-to-reach corner of a module. They should also not initially worry about code not covered by the requisite test module -- e.g. importlib only being tested by test_importlib -- and instead worry about testing as a whole from the entire CPython test suite.