Bug 21416: 'gr' option missing from ZEBRA_LANGUAGE options in koha-sites.conf

Bug 21416: 'gr' option missing from ZEBRA_LANGUAGE options in koha-sites.conf

'gr' (as in 'Greek') is missing from the ZEBRA_LANGUAGE options
in debian/templates/koha-sites.conf. This patch fixes that.

Test plan:
1) view koha-sites.conf and notice how 'gr' is missing from the
   ZEBRA_LANGUAGE options
2) apply the patch
3) observe that koha-sites.conf now includes 'gr' as an option

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>