Tags: HiveTechDev/HoloToolkit-Unity

Adding HoloToolkit-Unity package for release tag version v1.5.5.0

Release notes:
https://github.com/Microsoft/HoloToolkit-Unity/releases/tag/1.5.5.0

Merge pull request microsoft#338 from keveleigh/master

Adding StopSpeaking() to TextToSpeechManager