refactor: no need to use matchers.stringify in shell.lua.
refactor: no need to use matchers.stringify in shell.lua.

* specl/shell.lua (matchers.exit): Use `tostring` instead of
importing matchers.stringify into `q`.

Signed-off-by: Gary V. Vaughan <f9023000f29773649f3850298becb9544b5fd6a9@gnu.org>

gvvaughan/specl