Parity POA for MAC. Fix demo-spec.json

[Dave-Whiffin]

Launching parity POA test chain on mac results in the error below. The demo spec json seems to be invalid. I think this is an oversight for mac as the other parity test chains have a different json file.

Spec json is invalid: unknown field gasLimitBoundDivisor, expected one of stepDuration, validators, startStep, validateScoreTransition, validateStepTransition, immediateTransitions, blockReward, blockRewardContractTransition, blockRewardContractAddress, blockRewardContractCode, maximumUncleCountTransition, maximumUncleCount, emptyStepsTransition, maximumEmptySteps, strictEmptyStepsTransition at line 6 column 39

https://github.com/paritytech/parity-ethereum/issues/6916