rockspec: depend on at least stdlib 41.2.0.
rockspec: depend on at least stdlib 41.2.0.

Now that we're actively using latest features from the stdlib
debug library, be more careful about ensuring a sufficiently
compatible version is available.
* rockspec.conf (dependencies): Depend on a stdlib between
41.2.0, the earliest release with the debug apis we need, and
42.0.0, the next version that has backwards incompatible changes.

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

gvvaughan/specl