add jdk 10 to travis build matrix
add jdk 10 to travis build matrix

https://docs.travis-ci.com/user/languages/java/#using-java-10-and-later

unfortunately openjdk11 fails at present in contrast to the
documentation

ancho/jbake,jbake-org/jbake,jonbullock/jbake,ancho/jbake,jbake-org/jbake,jonbullock/jbake