travis: bump stable JRuby version to 1.7.19
travis: bump stable JRuby version to 1.7.19

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

killbill/killbill-litle-plugin,killbill/killbill-litle-plugin,killbill/killbill-litle-plugin