build: Include phpspec/prophecy dependency

Author: DannyvdSluijs

composer.json

@@ -34,7 +34,8 @@
     "require-dev": {
         "friendsofphp/php-cs-fixer": "~2.2.20 || ~2.19.0",
         "json-schema/json-schema-test-suite": "1.2.0",
-        "phpunit/phpunit": "^8.5"
+        "phpunit/phpunit": "^8.5",
+        "phpspec/prophecy": "^1.19"
     },
     "extra": {
         "branch-alias": {