update timestamp min in generated schema

an apparent omission from #648

Author: thekaveman

policy/policy.json

@@ -545,7 +545,7 @@
       ],
       "description": "Integer milliseconds since Unix epoch",
       "multipleOf": 1.0,
-      "minimum": 0
+      "minimum": 1514764800000
     },
     "null_uuid_array": {
       "$id": "#/definitions/null_uuid_array",