Add satooshi/php-coveralls dependency to Composer configuration

Author: niels-nijens

composer.json

@@ -23,7 +23,8 @@
         "twig/twig": "^1.24"
     },
     "require-dev": {
-        "phpunit/phpunit": "^4.8"
+        "phpunit/phpunit": "^4.8",
+        "satooshi/php-coveralls": "^1.0"
     },
     "autoload": {
         "psr-4": {