Revise embed code for Coveralls badge

Author: jmikola

README.md

@@ -2,7 +2,7 @@
 
 [![Travis Build Status](https://api.travis-ci.org/mongodb/mongo-php-driver.png?branch=master)](https://travis-ci.org/mongodb/mongo-php-driver)
 [![AppVeyor Build Status](https://ci.appveyor.com/api/projects/status/gbd3t99ucib5n8sf?svg=true)](https://ci.appveyor.com/project/jmikola/mongo-php-driver)
-[![Coverage Status](https://coveralls.io/repos/mongodb/mongo-php-driver/badge.svg?branch=master&service=github)](https://coveralls.io/github/mongodb/mongo-php-driver?branch=master)
+[![Coverage Status](https://coveralls.io/repos/github/mongodb/mongo-php-driver/badge.svg?branch=master)](https://coveralls.io/github/mongodb/mongo-php-driver?branch=master)
 
 This extension is developed atop the
 [libmongoc](https://github.com/mongodb/mongo-c-driver) and