Make tests pass with Java 8

This PR makes `org.graylog2.inputs.misc.jsonpath.SelectorTest` work with Java 8, disables doclint with Java 8 (see http://blog.joda.org/2014/02/turning-off-doclint-in-jdk-8-javadoc.html), 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).