Skip the identity test in VTK 5.x for object cache.
Skip the identity test in VTK 5.x for object cache.

The hash identity is related to a bug fix in VTK 6.x onwards. Hence skipped for
5.x.

dmsurti/mayavi,liulion/mayavi,dmsurti/mayavi,liulion/mayavi,alexandreleroux/mayavi,alexandreleroux/mayavi