Bump version to 4.2

Summary: Bump the version to 4.2 ( the unreleased version ), so that when fbcode_unittests run it can differentiate between old and new APIs

Test Plan: make check

Reviewers: sdong, yhchiang, igor

Reviewed By: igor

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D49041

include/rocksdb/version.h

@@ -5,7 +5,7 @@
 #pragma once
 
 #define ROCKSDB_MAJOR 4
-#define ROCKSDB_MINOR 1
+#define ROCKSDB_MINOR 2
 #define ROCKSDB_PATCH 0
 
 // Do not use these. We made the mistake of declaring macros starting with