Add PHPUnit development dependency

composer.json

@@ -13,6 +13,9 @@
         "php": ">=5.4",
         "ext-mongodb": "^1.1.0"
     },
+    "require-dev": {
+        "phpunit/phpunit": "^4.8"
+    },
     "autoload": {
         "psr-4": { "MongoDB\\": "src/" },
         "files": [ "src/functions.php" ]