Use just OpenJDK in Travis CI
Use just OpenJDK in Travis CI

Replace OracleJDK with OpenJDK, the former is no longer included by
default in newer dists (Xenial).

Signed-off-by: thc202 <43af82ef30812a20fbcf38b99a6f72cdd3a89ac1@gmail.com>

zaproxy/zap-api-java