Run a Gradle build after the Maven build

Eventually I'll want to swap that order, so that we know that Gradle
builds work even without any help from Maven build setup logic.  For
now, though, I just want to test whether the Gradle build works at
all.

.travis.yml

@@ -16,7 +16,9 @@ before_install:
 - export PATH=$M2_HOME/bin:$PATH
 install:
 - mvn clean verify -DskipTests=true -B -q
+- ./gradlew compileJava
 script:
+- xvfb ./gradlew build
 - xvfb-run mvn clean install -B -q
 - bash ./dependent-projects-trigger.sh $AUTH_TOKEN
 - mvn javadoc:aggregate -B -q