Add prophecy as dev dependency for better mocking

Author: Jamie Hannaford

composer.json

@@ -33,6 +33,7 @@
     },
     "require-dev" : {
         "phpunit/phpunit": "4.3.*",
+        "phpspec/prophecy": "~1.4",
         "satooshi/php-coveralls": "0.6.*@dev",
         "jakub-onderka/php-parallel-lint": "0.*",
         "fabpot/php-cs-fixer": "1.0.*@dev",