gemspec: update killbill gem to 3.1.11
gemspec: update killbill gem to 3.1.11

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

SquareMike/killbill-cybersource-plugin,killbill/killbill-cybersource-plugin,SquareMike/killbill-cybersource-plugin,killbill/killbill-cybersource-plugin,SquareMike/killbill-cybersource-plugin,killbill/killbill-cybersource-plugin