remove deprecated dep

Author: Jamie Hannaford

composer.json

@@ -27,7 +27,6 @@
     },
     "require-dev": {
         "phpunit/phpunit": "~4.0",
-        "phpspec/prophecy-phpunit": "~1.0",
         "sami/sami": "dev-master",
         "psr/log": "~1.0",
         "satooshi/php-coveralls": "~1.0",