Updating HISTORY.md

Summary: Added 4.3.0 version

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HISTORY.md

 # Rocksdb Change Log
 
-## Unreleased
+## 4.3.0 (12/8/2015)
 ### New Features
 * CompactionFilter has new member function called IgnoreSnapshots which allows CompactionFilter to be called even if there are snapshots later than the key.
 * RocksDB will now persist options under the same directory as the RocksDB database on successful DB::Open, CreateColumnFamily, DropColumnFamily, and SetOptions.