travis: Workaround broken Travis CI java configuration.

Travis CI seems to have broken /usr/bin/java in a recent
infrastructure change.

tools/travis/setup-production

@@ -15,6 +15,10 @@ tools/provision.py --travis --production-travis
 cp -a tools/travis/success-http-headers.txt ~/
 source tools/travis/activate-venv
 
+# Force OpenJDK 8 JRE. This is a workaround for Travis CI having
+# broken their java install, but also nicely provides consistency.
+sudo ln -nsf /usr/lib/jvm/java-8-openjdk-amd64/jre/bin/java /usr/bin/java
+
 py_version="$(python -c 'import sys; print(sys.version_info[0])')"
 if [ "$py_version" = 2 ]; then
     build_options=()