add integration test

As discussed in https://github.com/jubatus/jubatus/issues/856, I implemented integration test for jubadump, which validates that:

* jubadump exits with status code 0
* jubadump prints valid JSON

I'll add this to CI once this patch is merged.

To run the test, simply run:

```
$ jubatest --testcase integration_test
```

Note: develop branch of jubatest is required for now.  I'm currently working on merging it to master branch.