update php-mongodb extension version requirement to 2.x

Author: samuelrajan747

composer.json

@@ -19,7 +19,7 @@
     ],
     "require": {
         "yiisoft/yii2": "~2.0.39",
-        "ext-mongodb": ">=1.20.1",
+        "ext-mongodb": ">=2.0.0",
         "paragonie/random_compat": ">=1"
     },
     "require-dev": {