Vacuum gripper plugin uses walltime instead of simtime

[Steven Gray]

Vacuum gripper plugin OnUpdate() is using common::Time::GetWallTime() instead of world->SimTime() when checking to see if it should update.

https://bitbucket.org/osrf/ariac/src/master/osrf_gear/src/VacuumGripperPlugin.cc lines 393, 447, 516

Not a huge deal unless someone manages to run over 1.0x RTF, but still good to fix.

[Shane Loretz]

Thanks for the issue! Fixed by pull request #144