message: update news item for v1.8.3 release
diff: News, Updates, and Announcements
  ================================
  -Mar 13, 2019: Version v1.8.3 now released on GitHub
  +Mar 22, 2019: Version v1.8.3 now released on GitHub
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  New release version 1.8.3 is now available on the [OSPRay GitHub
  