Pin prophecy to 1.6 to avoid scalar type declaration error

Author: Max Baldanza

composer.json

@@ -7,6 +7,7 @@
     "require": {
         "php": "^7.0",
         "phpspec/phpspec": "^4.0 <4.1.0",
+        "phpspec/prophecy": "^1.6",
         "symfony/dependency-injection": "^3.2",
         "symfony/config": "^3.2",
         "shanethehat/pretty-xml": "~1.0.2"