Add dev requirement for helmich/mongomock

Author: chadicus

composer.json

@@ -39,6 +39,7 @@
         "subjective-php/psr-cache-mongodb": "Used for Caching with Mongo"
     },
     "require-dev": {
+        "helmich/mongomock": "^2.1",
         "php-coveralls/php-coveralls": "^2.1",
         "phpunit/phpunit": "^6.5.2",
         "squizlabs/php_codesniffer": "^3.2",