Disallow chaining of Promise.start()

This kind of chaining can open the door to a race condition where an
older disposable (from start().signal.observe) can overwrite a newer
disposable (set within the observation closure).
Merge branch 'swift-development' into dont-chain-me-bro

Conflicts:
	ReactiveCocoa/Promise.swift
Merge pull request #1444 from ReactiveCocoa/dont-chain-me-bro

Disallow chaining of Promise.start()