Prerelease 2018-06-11

This is a slightly different prerelease approach leading up to Roact 1.0.

This release rolls up all of the changes since prerelease 2 a couple months ago:

• Renamed Roact.reify to Roact.mount and Roact.teardown to Roact.unmount (#82)
  • The old methods are still present as aliases, but will output a warning when used.
• Added Roact.Change for subscribing to GetPropertyChangedSignal (#51)
• Added the static lifecycle method getDerivedStateFromProps (#57)
• Allow canceling render by returning nil from setState callback (#64)
• Added defaultProps value on stateful components to define values for props that aren't specified (#79)
• Added getElementTraceback (#81, #93)
• Added createRef (#70, #92)
• Ref switching now occurs in one pass, which should fix edge cases where the result of a ref is nil, especially in property changed events (#98)

The next release should hopefully be 1.0!