gemspec: update to killbill gem 3.1.12
gemspec: update to killbill gem 3.1.12

Signed-off-by: Pierre-Alexandre Meyer <ff019a5748a52b5641624af88a54a2f0e46a9fb5@mouraf.org>

killbill/killbill-payu-latam-plugin,killbill/killbill-payu-latam-plugin,killbill/killbill-payu-latam-plugin