Make tests pass with Java 8

This PR makes `org.graylog2.inputs.misc.jsonpath.SelectorTest` work with Java 8, makes the new Javadoc Linter happy, and adds `oraclejdk8` back to the Travis CI configuration.

The test failed because `HashMap` doesn't guarantee the iteration order and this promptly changed somewhere between Java 7 (7u60) and Java 8 (8u5).