use openjdk7 instead of oraclejdk7
use openjdk7 instead of oraclejdk7

travis-ci no longer support oraclejdk7

- https://github.com/travis-ci/travis-ci/issues/7884#issuecomment-308451879
- http://www.webupd8.org/2017/06/why-oracle-java-7-and-6-installers-no.html

sbt/sbt-appengine