Update release version to 6.8 (#6450)

Summary:
Update release version to 6.8
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6450

Test Plan: no code change

Differential Revision: D20071889

Pulled By: cheng-chang

fbshipit-source-id: 91450aae09b201926469ff32f59ed436366f3b74

HISTORY.md

 # Rocksdb Change Log
-## Unreleased
+## 6.8.0 (02/24/2020)
 ### Java API Changes
 * Major breaking changes to Java comparators, toward standardizing on ByteBuffer for performant, locale-neutral operations on keys (#6252).
 * Added overloads of common API methods using direct ByteBuffers for keys and values (#2283).

include/rocksdb/version.h

@@ -5,7 +5,7 @@
 #pragma once
 
 #define ROCKSDB_MAJOR 6
-#define ROCKSDB_MINOR 7
+#define ROCKSDB_MINOR 8
 #define ROCKSDB_PATCH 0
 
 // Do not use these. We made the mistake of declaring macros starting with