[travis] Only run javadocAll on Java 11 or higher

It appears that 'javadoc' from older JREs is not able to consume the
javadoc generated by newer JREs:

javadoc: warning - Error fetching URL: https://jxmpp.org/releases/0.7.0-alpha6/javadoc/
javadoc: warning - Error fetching URL: https://minidns.org/releases/0.4.0-alpha5/javadoc/

.travis.yml

@@ -35,7 +35,16 @@ install: gradle assemble --stacktrace
 # functional. Which hasn't always be the case in the past, see
 # 90cbcaebc7a89f4f771f733a33ac9f389df85be2
 # Also run javadocAll to ensure it works.
-script: gradle check publishToMavenLocal javadocAll --stacktrace
+script:
+  - |
+    JAVAC_MAJOR_VERSION=$(javac -version | sed -E 's/javac ([[:digit:]]+).*/\1/')
+    GRADLE_TASKS=()
+    GRADLE_TASKS+=(check)
+    GRADLE_TASKS+=(publishToMavenLocal)
+    if [[ ${JAVAC_MAJOR_VERSION} -ge 11 ]]; then
+      GRADLE_TASKS+=(javadocAll)
+    fi
+    gradle ${GRADLE_TASKS[@]} --stacktrace
 
 after_success:
   - JAVAC_VERSION=$((javac -version) 2>&1)