travis: Add javadocAll to 'script' gradle targets

to ensure that javadocAll works correctly. A nice side-effect, this
also ensures that the package-info.java symbolic links are still
correct and haven't been replaced by some IDE or editor with the
content of the link target.

.travis.yml

@@ -35,7 +35,8 @@ install: gradle assemble --stacktrace
 # archive to additionaly test if artifact creation is
 # functional. Which hasn't always be the case in the past, see
 # 90cbcaebc7a89f4f771f733a33ac9f389df85be2
-script: gradle check install --stacktrace
+# Also run javadocAll to ensure it works.
+script: gradle check install javadocAll --stacktrace
 
 after_success:
   - JAVAC_VERSION=$((javac -version) 2>&1)