Bump minimum cmake version to 3.5 to avoid deprecation warning

Building msgpack gives currently gives the following warning:
"CMake Deprecation Warning at CMakeLists.txt:1 (CMAKE_MINIMUM_REQUIRED):
  Compatibility with CMake < 3.5 will be removed from a future version of
  CMake.

  Update the VERSION argument <min> value or use a ...<max> suffix to tell
  CMake that the project does not need compatibility with older versions."

CMakeLists.txt

@@ -1,9 +1,4 @@
-CMAKE_MINIMUM_REQUIRED (VERSION 2.8.12)
-
-IF ((CMAKE_VERSION VERSION_GREATER 3.1) OR
-    (CMAKE_VERSION VERSION_EQUAL 3.1))
-    CMAKE_POLICY(SET CMP0054 NEW)
-ENDIF ()
+CMAKE_MINIMUM_REQUIRED (VERSION 3.5)
 
 OPTION (MSGPACK_BUILD_TESTS "Build msgpack tests." OFF)
 OPTION (MSGPACK_GEN_COVERAGE "Enable running gcov to get a test coverage report." OFF)