SignalProducerSpec: fixed compilation now that order of parameters is enforced
SignalProducerSpec: fixed compilation now that order of parameters is enforced

See [SR-0060](https://github.com/apple/swift-evolution/blob/master/proposals/0060-defaulted-parameter-order.md) for details.

sandyway/ReactiveCocoa,calebd/ReactiveCocoa,victorlin/ReactiveCocoa,ohwutup/ReactiveCocoa,walkingsmarts/ReactiveCocoa,victorlin/ReactiveCocoa,ikesyo/ReactiveCocoa,ikesyo/ReactiveCocoa,ReactiveCocoa/ReactiveSwift,sandyway/ReactiveCocoa,calebd/ReactiveCocoa,ohwutup/ReactiveCocoa,calebd/ReactiveCocoa,walkingsmarts/ReactiveCocoa,walkingsmarts/ReactiveCocoa,ReactiveCocoa/ReactiveSwift,victorlin/ReactiveCocoa,sandyway/ReactiveCocoa,ReactiveCocoa/ReactiveSwift,ohwutup/ReactiveCocoa