travis: upgrade JRuby to 1.7.26
travis: upgrade JRuby to 1.7.26

See https://github.com/killbill/killbill-oss-parent/blob/83732ead30d0ac798d809a4ce8fb4147ee924ed8/pom.xml#L739.

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

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