JAVA-2727: Also disable javadoc plugin for test phase

api-plumber-doclet runs on the process-classes phase, and it's currently
not compatible with Java 11 (JAVA-2610).

Author: olim7t

.travis.yml

@@ -16,7 +16,7 @@ before_install:
   - jdk_switcher use openjdk8
   - ./install-snapshots.sh
 install: mvn install -DskipTests=true -Dmaven.javadoc.skip=true -B -V
-script: mvn test -Djacoco.skip=true -Dmaven.test.failure.ignore=true -B -V
+script: mvn test -Djacoco.skip=true -Dmaven.test.failure.ignore=true -Dmaven.javadoc.skip=true -B -V
 cache:
   directories:
     - $HOME/.m2