AWS Key management plugin does not build on Centos7.

Building AWS C++ SDK as external project needs CMake 2.8.12
Centos7  out of the box has 2.8.11, thus the build fails.

Fixed check for required CMake version.

plugin/aws_key_management/CMakeLists.txt

@@ -68,7 +68,8 @@ IF(AWS_CPP_SDK_CORE AND AWS_CPP_SDK_KMS AND HAVE_AWS_HEADERS)
   SET(AWS_SDK_LIBS ${AWS_CPP_SDK_CORE} ${AWS_CPP_SDK_KMS})
 ELSE()
   # Build from source, using ExternalProject_Add
-  IF(CMAKE_VERSION VERSION_LESS "2.8.8")
+  # AWS C++ SDK requires cmake 2.8.12
+  IF(CMAKE_VERSION VERSION_LESS "2.8.12")
     SKIP_AWS_PLUGIN("CMake is too old")
   ENDIF()
   FIND_PACKAGE(Git)