Allow puppetlabs/stdlib 6.x (#93)

stdlib 6.0.0 is due to be released soon.
This module has already dropped support for puppet 4 and/or doesn't
use the updated stdlib `merge` function.

See
https://github.com/puppetlabs/puppetlabs-stdlib/blob/217068f97edd2396ce37ac8c1bd7e23da621d6be/CHANGELOG.md#supported-release-600

metadata.json

@@ -10,7 +10,7 @@
   "dependencies": [
     {
       "name": "puppetlabs-stdlib",
-      "version_requirement": ">= 4.13.1 < 6.0.0"
+      "version_requirement": ">= 4.18.0 < 7.0.0"
     }
   ],
   "requirements": [