Only use GRIPPER_OPEN_chull meshes.

As far as I can tell, the Robotiq hand description does not contain any
non-convex-hull, single-mesh representations of the entire hand. I've edited
`atlas_minimal_contact.urdf` to use GRIPPER_OPEN_chull throughout.

Resolves #435
Merge pull request #531 from avalenzu/akv-missing-atlas-gripper-meshes

Only use GRIPPER_OPEN_chull meshes.