Changed coverage badge to coveralls from shields.io

Unfortunately badges/shield#168 means that the images from shields.io for coveralls are unreliable, so until that's resolved I'm switching back to pulling the coveralls badge directly.

Author: tedivm

README.md

@@ -2,7 +2,7 @@
 
 [![License](http://img.shields.io/packagist/l/tedivm/JShrink.svg)](https://github.com/tedivm/JShrink/blob/master/LICENSE)
 [![Latest Stable Version](http://img.shields.io/github/release/tedivm/JShrink.svg)](https://packagist.org/packages/tedivm/JShrink)
-[![Coverage Status](http://img.shields.io/coveralls/tedivm/JShrink.svg)](https://coveralls.io/r/tedivm/JShrink?branch=master)
+[![Coverage Status](https://coveralls.io/repos/tedivm/JShrink/badge.png?branch=master)](https://coveralls.io/r/tedivm/JShrink?branch=master)
 [![Total Downloads](http://img.shields.io/packagist/dt/tedivm/jshrink.svg)](https://packagist.org/packages/tedivm/JShrink)