fix: carbon-relay unable to start with relay_method=aggregated-consistent-hashing

without this fix, carbon-relay will fail as follows:
Traceback (most recent call last):
  File "/opt/graphite/bin/carbon-relay.py", line 32, in <module>
    run_twistd_plugin(__file__)
  File "/opt/graphite/lib/carbon/util.py", line 91, in run_twistd_plugin
    runApp(config)
  File "/usr/lib/python2.7/dist-packages/twisted/scripts/twistd.py", line 23, in runApp
    _SomeApplicationRunner(config).run()
  File "/usr/lib/python2.7/dist-packages/twisted/application/app.py", line 376, in run
    self.application = self.createOrGetApplication()
  File "/usr/lib/python2.7/dist-packages/twisted/application/app.py", line 436, in createOrGetApplication
    ser = plg.makeService(self.config.subOptions)
  File "/opt/graphite/lib/twisted/plugins/carbon_relay_plugin.py", line 21, in makeService
    return service.createRelayService(options)
  File "/opt/graphite/lib/carbon/service.py", line 187, in createRelayService
    RuleManager.read_from(settings["aggregation-rules"])
  File "/opt/graphite/lib/carbon/aggregator/rules.py", line 21, in read_from
    self.read_rules()
  File "/opt/graphite/lib/carbon/aggregator/rules.py", line 25, in read_rules
    if not exists(self.rules_file):
  File "/usr/lib/python2.7/genericpath.py", line 18, in exists
    os.stat(path)
TypeError: coercing to Unicode: need string or buffer, NoneType found