feat(pie): allow installing mongodb extension using php/pie

Author: rvanlaak

composer.json

@@ -119,6 +119,7 @@
         "willdurand/negotiation": "^3.1"
     },
     "require-dev": {
+        "ext-mongodb": "^1.21 || ^2.0",
         "behat/behat": "^3.11",
         "behat/mink": "^1.9",
         "doctrine/cache": "^1.11 || ^2.1",