Add requirement for mongodb extension

Author: chadicus

composer.json

@@ -25,7 +25,8 @@
     },
     "require": {
         "php": "^7.0",
-	    "mongodb/mongodb": "^1.1"
+        "ext-mongodb": "*",
+        "mongodb/mongodb": "^1.1"
     },
     "require-dev": {
         "phpunit/phpunit": "^6.5.2",