Fix guacamaole-client's build stopped working (#506)

The following error was returned by APT:
The following packages have unmet dependencies:
 openjdk-8-jdk : Depends: openjdk-8-jre (= 8u121-b13-1~bpo8+1) but it is not going to be installed
                 Depends: openjdk-8-jdk-headless (= 8u121-b13-1~bpo8+1) but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

The reason for the sudden build breaking is unknown. Upgrading from tomcat 8.5.4 to 8.5.13 fixed the issue.