[Java] Failed to build document with OpenJDK 11

It reports the following error:

```

[ERROR] Exit code: 1 - javadoc: error - The code being documented uses modules but the packages defined in http://docs.oracle.com/javase/8/docs/api/ are in the unnamed module.
```

See also: https://travis-ci.org/kou/arrow/jobs/551254733#L1453

This branch just enables Javadoc with OpenJDK 11: https://github.com/kou/arrow/commit/1eeded4b9d18d474721733751f57392cee766004.diff

```

diff --git a/.travis.yml b/.travis.yml
index 5dc901561e8..1d6ba86dc2d 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -225,6 +225,7 @@ matrix:
- if [ $ARROW_CI_JAVA_AFFECTED != "1" ]; then exit; fi
script:
- $TRAVIS_BUILD_DIR/ci/travis_script_java.sh
+ - $TRAVIS_BUILD_DIR/ci/travis_script_javadoc.sh
- name: "Integration w/ OpenJDK 8, conda-forge toolchain"
language: java
os: linux

```

**Reporter**: [Kouhei Sutou](https://issues.apache.org/jira/browse/ARROW-5764) / @kou

_{**Note**: *This issue was originally created as [ARROW-5764](https://issues.apache.org/jira/browse/ARROW-5764). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Start with the OpenJDK 11 Javadoc job in .travis.yml and inspect ci/travis_script_javadoc.sh. Reproduce the reported Javadoc command and error using the referenced Travis job, then identify the configuration change needed for the documented Java packages. Done means the OpenJDK 11 Javadoc step completes successfully.

索引モデルが issue の本文から書いたものです。