v1.5.5: [ciTravis] continuous integration on Travis

Release Notes
This is the version of Geogram shipped with (upcoming) Graphite3-1.5.5
Change Log

• Updated to ImGui 1.53 (has Styles)
• Several functions made virtual in GLUP viewer GUI (for Graphite)
• Minor bugfixes and warning fixes
  Download:

https://gforge.inria.fr/frs/?group_id=5833