v1.9.0

• Feature: Improve forward compatibility with upcoming Promise v3 API.
  (#54 and #55 by @clue)

• Minor documentation improvements for upcoming Promise v3.
  (#58 by @clue and #56 by @SimonFrings)

• Improve test suite, fix legacy HHVM build by downgrading Composer.
  (#57 by @SimonFrings)