Logging must be configured to be used

So after releasing https://github.com/edx/diff-cover/pull/108 I did some manual testing. Installing the package and testing the error messages.

I noticed I got: ``No handlers could be found for logger "diff_cover.tool"``

That is when I realized diff-cover was using the logging module but never configured it. This PR adds the call to basic config with a format that is just the message.

@wedaly, @jzoldak, @clytwynec, @benpatterson Does this affect you guys at all? Was there a reason logging was not configured or is this just an oversight?