Fix stdlib version in metadata

puppetlabs-stdlib is now using the 5.1 version, so we should fix the
stdlib version in metadata.

Change-Id: If4b34528da161a1cc9acf9349005c01724e3db20

metadata.json

@@ -60,11 +60,11 @@
         },
         {
             "name": "puppetlabs/stdlib",
-            "version_requirement": ">=4.0.0 <5.0.0"
+            "version_requirement": ">=5.0.0 <6.0.0"
         },
         {
             "name": "puppet/python",
             "version_requirement": ">=0.0.1 <2.0.0"
         }
     ]
-}
+}