Displays info about installed versions

This is a variation on https://github.com/pypa/pip/pull/2207 that
doesn't write the output in `package_name==version` format, because
@dstufft had some reservations about using that format and then people
getting accustomed to it.

It also doesn't display the version if it can't get one for some reason.
I'm hoping this plays nicer with the tests where often the version comes
out as `None`.