message: create news item for release
diff: News, Updates, and Announcements
  ================================
  +Dec 5, 2018: Version v1.7.3 now released on GitHub
  +---------------------------------------------------
  +
  +New release version 1.7.3 is now available on the [OSPRay GitHub
  +page](https://github.com/ospray/OSPRay/releases/v1.7.3).
  +
  +
  Nov 9, 2018: Version v1.7.2 now released on GitHub
  ---------------------------------------------------
  