As can be seen by this job (and all the other jobs after) the generation of Javadoc seems to hang the entire build when run on OpenJDK 11. This worked on the previous build for that branch which was based on the same commit as the failing job (and the same behavior can be seen on the most recent builds for that project), which is why I suspect an environment problem.
Running on OpenJDK11 locally gives error “[ERROR] Exit code: 1 - javadoc: error - The code being documented uses modules but the packages defined in https://docs.oracle.com/javase/8/docs/api/ are in the unnamed module.”, which seems related to this bug, but for some reason Travis truncates the log before the error message is shown.