Bug 14427: Use Koha::version instead of C4::Context->KOHAVERSION

Bug 14427: Use Koha::version instead of C4::Context->KOHAVERSION

While testing bug 9006, I discovered this gem in
installer/install.pl

TEST PLAN
---------
 1) back up DB
 2) drop db and create db
 3) git reset --hard origin/3.20.x
 4) run web installer and all sample data
 5) git reset --hard origin/master
 6) empty your koha intranet error log
 7) run web installer to upgrade
    -- there will be warnings regarding regex around
       installer/install.pl line 328 or so.
       grep "\$kohaversion" ~/koha-dev/var/logs/koha-error_log
 8) repeat steps 2-6
 9) apply this patch
10) run web installer to upgrade
    -- no regex warnings.
       grep "\$kohaversion" ~/koha-dev/var/logs/koha-error_log
11) koha qa test tools

Signed-off-by: Jonathan Druart <jonathan.druart@koha-community.org>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@unc.edu.ar>