Lower dependency on Storable to 2.20 in PerlDependencies.pm

Lower dependency on Storable to 2.20 in PerlDependencies.pm

This patch lowers the dependency on the "Storable" Perl package
(also known as module/distribution) from version 2.21 to 2.20

This patch created today (14-Apr-2011) does, in PerlDependencies.pm,
the same as a previous patch dated 30-Mar-2010 from Lars Wirzenius
did, in Makefile.PL :

[Koha-patches] [PATCH] Lower dependency on Storable to 2.20.
http://lists.koha-community.org/pipermail/koha-patches/2010-March/011560.html

The reason for this patch is the same that Lars gave for his:

" The Debian squeeze system has only 2.20, and the 2.21 does not seem
 to be required for all tests to pass. Also, checked with
Galen Charlton and Henri-Damien LAURENT (who added the 2.21 dependency)
on IRC that 2.21 did not have anything that was required, compared
to 2.20. "

I believe this patch should be applied to the "master" branch
and should also be "cherry-picked" for the "3.2.x" branch.

Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>