Fixed Urchin invocation problem in zsh and dash.  

The - currently - pointless invocations of Urchin itself from the various shells have been removed, pending the amendment to Urchin that will allow true cross-shell tests (see #551).

This fixes #565.

Instead of the caveat, could we just add the `-s $SHELL` now, and it would theoretically be a noop until the PR is merged?