MINOR: Fix warnings for -Werror

Signed-off-by: Arjun Satish <arjun@confluent.io>

Author: wicknicks

src/main/java/io/confluent/connect/elasticsearch/jest/JestElasticsearchClient.java

@@ -222,7 +222,7 @@ private boolean indexExists(String index) {
     if (indexCache.contains(index)) {
       return true;
     }
-    Action action = new IndicesExists.Builder(index).build();
+    Action<?> action = new IndicesExists.Builder(index).build();
     try {
       JestResult result = client.execute(action);
       return result.isSucceeded();