Update history and version for 6.1

Summary: Pull Request resolved: https://github.com/facebook/rocksdb/pull/5119

Differential Revision: D14645216

Pulled By: gfosco

fbshipit-source-id: f7c83dca22c2486fc5d8697b61638c382889d073

HISTORY.md

 # Rocksdb Change Log
-## Unreleased
+
+## 6.1.0 (3/27/2019)
 ### New Features
 * Introduce two more stats levels, kExceptHistogramOrTimers and kExceptTimers.
 * Added a feature to perform data-block sampling for compressibility, and report stats to user.

include/rocksdb/version.h

@@ -5,7 +5,7 @@
 #pragma once
 
 #define ROCKSDB_MAJOR 6
-#define ROCKSDB_MINOR 0
+#define ROCKSDB_MINOR 1
 #define ROCKSDB_PATCH 0
 
 // Do not use these. We made the mistake of declaring macros starting with