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.

`urchin -f test/install_script -s dash` works fine locally, it's just got naive option parsing, so the `-f` has to go first. Presumably with the PR, option ordering wouldn't matter.