Update cmake_minimum_required to 2.8.12.

Summary:
Hello,

current master branch declares cmake_minimum_required (VERSION 2.8.11)
but cmake gives the following error:

[  6%] CMake Error at CMakeLists.txt:658 (install):
  install TARGETS given unknown argument "INCLUDES".

CMake Error at src/CMakeLists.txt:658 (install): install TARGETS given unknown argument "INCLUDES".

because this argument not supported on CMake versions prior 2.8.12
Closes https://github.com/facebook/rocksdb/pull/2904

Differential Revision: D5863430

Pulled By: yiwu-arbug

fbshipit-source-id: 0f7230e080add472ad4b87836b3104ea0b971a38

CMakeLists.txt

@@ -32,7 +32,7 @@
 # 3. cmake ..
 # 4. make -j
 
-cmake_minimum_required(VERSION 2.8.11)
+cmake_minimum_required(VERSION 2.8.12)
 project(rocksdb)
 
 if(POLICY CMP0042)