fix: fix a deprecation warning from jsonschema 3.x (#946)

For example, in https://travis-ci.org/awslabs/serverless-application-model/jobs/538917819:

/home/travis/build/awslabs/serverless-application-model/.tox/py36/lib/python3.6/site-packages/jsonschema/validators.py:893: DeprecationWarning: The metaschema specified by $schema was not found. Using the latest draft to validate, but this will raise an error in the future.
  cls = validator_for(schema)

samtranslator/policy_templates_data/schema.json

@@ -1,5 +1,5 @@
 {
-  "$schema": "http://json-schema.org/schema#",
+  "$schema": "http://json-schema.org/draft-07/schema#",
   "title": "Schema for AWS SAM Policy Templates",
 
   "definitions": {