Use Github URL for json schema

[otienoanyango]

in validator.py GitLab URL is used for funpdbe schema. Hoping GH and GL haven't diverged, it is probably a good time to change the URL to GH or make it dynamic over env variable.
self.json_url = "https://raw.githubusercontent.com/PDBe-KB/funpdbe-schema/master/funpdbe_example.json"