Fixed compatibility issue with ES 1.3 and 1.4

The API changed in version 1.3 and 1.4, see the following documents. More precisely, the REST API is

http://localhost:9200/_nodes/stats

instead of

http://localhost:9200/_cluster/nodes/stats

References:

1) http://www.elasticsearch.org/guide/en/elasticsearch/reference/1.3/cluster-nodes-stats.html

2) http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/cluster-nodes-stats.html