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.

The original command worked fine on BSD-like system (including OSX), but didn't on Linux systems. The Travis tests happened not to be affected, because they explicitly specify the test suite to run. The revised command should be POSIX-compliant.