ignition-gazebo6 6.8.0 (#1862)
ignition-gazebo6 6.8.0 (#1862)

* ignition-gazebo6: update 6.8.0

* update  bottle.

Co-authored-by: Steve Peters <19cdd629ec3b4ea4663c77a4169548f370fdc02f@openrobotics.org>
osrf/homebrew-simulation