Bug 26848: Fix Readonly dependency in cpanfile

Bug 26848: Fix Readonly dependency in cpanfile

- It is a required module. If it is missing, Koha will not start
- Set the minimum version to 2.00. While not strictly required, it's the
version that states that "Deprecation of Readonly::XS as a requirement
for fast, readonly scalars is complete", it is available in Debian
oldoldstable (jessie, 8), and Readonly::XS was not even needed with Perl
> 5.8, which allows us to:
- Remove dependency on Readonly::XS

Signed-off-by: Mason James <mtj@kohaaloha.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>