[elasticearch] upgrade to version 2.4.0
[elasticearch] upgrade to version 2.4.0

This commit upgrades the Elasticsearch dependency from version 2.3.4 to
version 2.4.0.

brianfrankcooper/YCSB,zyguan/ycsb,brianfrankcooper/YCSB,leschekhomann/YCSB,brianfrankcooper/YCSB,brianfrankcooper/YCSB,cricket007/YCSB,cricket007/YCSB,ChristianNavolskyi/YCSB,cricket007/YCSB,madhurihn/YCSB_ToyDB,leschekhomann/YCSB,leschekhomann/YCSB,madhurihn/YCSB_ToyDB,ChristianNavolskyi/YCSB,madhurihn/YCSB_ToyDB,jaemyoun/YCSB,leschekhomann/YCSB,cricket007/YCSB,zyguan/ycsb,jaemyoun/YCSB,zyguan/ycsb,jaemyoun/YCSB,manolama/YCSB,zyguan/ycsb,jaemyoun/YCSB,ChristianNavolskyi/YCSB,madhurihn/YCSB_ToyDB,manolama/YCSB,manolama/YCSB,ChristianNavolskyi/YCSB,manolama/YCSB