move prophecy-phpunit to a dev dependency as it's not required other than to execute tests.

Author: steve.brazier

composer.json

@@ -30,7 +30,10 @@
         "symfony/console"          : "2.*",
         "psr/log": "1.0.0",
         "monolog/monolog": "1.6.0",
-        "pimple/pimple": "1.1.*",
+        "pimple/pimple": "1.1.*"
+    },
+
+    "require-dev": {
         "phpspec/prophecy-phpunit": "1.*"
     },