Integ failure with ss

Integration test failure spotted by Nick...

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ERROR: test_connections_by_ss
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Traceback (most recent call last):
  File "stem/test/integ/util/connection.py", line 49, in test_connections_by_ss
    self.check_resolver(Resolver.SS)
  File "stem/test/require.py", line 58, in wrapped
    return func(self, *args, **kwargs)
  File "stem/test/integ/util/connection.py", line 28, in check_resolver
    connections = get_connections(resolver, process_pid = runner.get_pid())
  File "stem/stem/util/connection.py", line 300, in get_connections
    raise IOError('No results found using: %s' % resolver_command)
OSError: No results found using: ss -nptu

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