Update history and version for future 5.18 release.

Summary: Pull Request resolved: https://github.com/facebook/rocksdb/pull/4669

Differential Revision: D13031522

Pulled By: gfosco

fbshipit-source-id: d09e655a9d5556594f195c5d1b786900932145ce

HISTORY.md

 # Rocksdb Change Log
 ## Unreleased
+
+## 5.18.0 (11/12/2018)
 ### New Features
 * Introduced `Memoryllocator`, which lets the user specify custom allocator for memory in block cache.
 * Introduced `PerfContextByLevel` as part of `PerfContext` which allows storing perf context at each level. Also replaced `__thread` with `thread_local` keyword for perf_context.

include/rocksdb/version.h

@@ -5,7 +5,7 @@
 #pragma once
 
 #define ROCKSDB_MAJOR 5
-#define ROCKSDB_MINOR 17
+#define ROCKSDB_MINOR 18
 #define ROCKSDB_PATCH 0
 
 // Do not use these. We made the mistake of declaring macros starting with