travis: enable JDK 1.8 builds only and add Bintray
travis: enable JDK 1.8 builds only and add Bintray

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

killbill/killbill-client-java,killbill/killbill-client-java