0.15.0

CHANGES:

• Add isValidElement (@r17x in
  #837)
• Add startTransition (@r17x in
  #838)
• Convert ReasonReactErrorBoundary to Reason instead of %raw JS. This has
  the benefit of skipping a hardcoded require('react') call (@anmonteiro in
  #839)
• Add CSS Box Alignment Module Level 3 (@davesnx in
  #847)
• Fix: Remove "unique key prop" warnings from multi-child fragment elements
  (@jchavarri in #852)