Fixed clamp_on_holder (original mesh is broken)

The clamp_on_holder mesh in the original has problems, and doesn't slice correctly in Simplfy3D – it falls into two pieces when removed from the bed! This is simply the same file passed through netfabb – so use this rather than the original if using Simplify3D.

Printed at Seaside 3D, UK. www.seaside3d.com

All design credit goes to the original designer.