Tests: remove _river index before exiting test

It could happen that when closing a test, some Lucene files are still opened.

```
ERROR   20.6s | WikipediaRiverTest.testWikipediaRiverFrench <<<
   > Throwable #1: java.lang.RuntimeException: MockDirectoryWrapper: cannot close: there are still open files: {_0.cfs=1, _1.cfs=1}
   >    at __randomizedtesting.SeedInfo.seed([BA6DF9B07D6F6CD0:845570F06FCD3422]:0)
   >    at org.apache.lucene.store.MockDirectoryWrapper.close(MockDirectoryWrapper.java:701)
   >    at org.elasticsearch.test.store.MockDirectoryHelper$ElasticsearchMockDirectoryWrapper.close(MockDirectoryHelper.java:140)
   >    at org.elasticsearch.test.hamcrest.ElasticsearchAssertions.assertAllFilesClosed(ElasticsearchAssertions.java:664)
   >    at org.elasticsearch.test.TestCluster.assertAfterTest(TestCluster.java:76)
   >    at org.elasticsearch.test.ElasticsearchIntegrationTest.afterInternal(ElasticsearchIntegrationTest.java:602)
   >    at org.elasticsearch.test.ElasticsearchIntegrationTest.after(ElasticsearchIntegrationTest.java:1720)
```

Cleaning the `_river` index should fix that.

Closes #36.
Update to elasticsearch 1.4.0.Beta1

Related to #27.