Update Coverall README image to use master branch.

:fork_and_knife: The previous image was the Coverall % for the `63978174-coveralls` branch. This changes the image to be for master branch. :sparkles: 

What it looks like:

[![Coverage Status](https://coveralls.io/repos/opscode/supermarket/badge.png?branch=master)](https://coveralls.io/r/opscode/supermarket?branch=master)