arcrc: Stub data to avoid crash on startup

On a fresh checkout, docker-compose up fails with:

Starting QA Report App
Traceback (most recent call last):
  File "./webhook.py", line 350, in <module>
    main()
  File "./webhook.py", line 334, in main
    connect_to_phab(config.get('arcrc'))
  File "/app/taskmanager.py", line 50, in connect_to_phab
    phabricator.ARCRC.update(json.load(f))
  File "/usr/lib/python3.7/json/__init__.py", line 296, in load
    parse_constant=parse_constant, object_pairs_hook=object_pairs_hook, **kw)
  File "/usr/lib/python3.7/json/__init__.py", line 348, in loads
    return _default_decoder.decode(s)
  File "/usr/lib/python3.7/json/decoder.py", line 337, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/lib/python3.7/json/decoder.py", line 355, in raw_decode
    raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)

By making the default arcrc valid enough the service now starts up just fine and it simply disables the Phabricator connection:

Phab API connection failed: ERR-INVALID-AUTH: API token "replace-me-with-an-actual-token!" has the wrong format. API tokens should be 32 characters long and begin with one of these prefixes: api, cli, clr.