message: add news item for v1.8.0
diff: News, Updates, and Announcements
  ================================
  +Feb 1, 2019: Version v1.8.0 now released on GitHub
  +---------------------------------------------------
  +
  +New release version 1.8.0 is now available on the [OSPRay GitHub
  +page](https://github.com/ospray/OSPRay/releases/v1.8.0).
  +
  +
  Dec 5, 2018: Version v1.7.3 now released on GitHub
  ---------------------------------------------------
  