BUG: VERSION_LESS_EQUAL introduced in cmake 3.7

Older versions of cmake, according to documentation:
https://cmake.org/cmake/help/v3.5/command/if.html , do not know
VERSION_LESS_EQUAL, just VERSION_LESS.

This leads to errors:

CMake Error at somewhere/jsoncpp/CMakeLists.txt:18 (if):
  if given arguments:

    "3.5.1" "VERSION_LESS_EQUAL" "3.13.1"

  Unknown arguments specified

Resolves: #866

Author: hjmjohnson

Diff for: CMakeLists.txt

@@ -13,9 +13,9 @@
 # continue to generate policy warnings "CMake Warning (dev)...Policy CMP0XXX is not set:"
 #
 set(JSONCPP_OLDEST_VALIDATED_POLICIES_VERSION "3.1.0")
-set(JSONCPP_NEWEST_VALIDATED_POLICIES_VERSION "3.13.1")
+set(JSONCPP_NEWEST_VALIDATED_POLICIES_VERSION "3.13.2")
 cmake_minimum_required(VERSION ${JSONCPP_OLDEST_VALIDATED_POLICIES_VERSION})
-if("${CMAKE_VERSION}" VERSION_LESS_EQUAL "${JSONCPP_NEWEST_VALIDATED_POLICIES_VERSION}")
+if("${CMAKE_VERSION}" VERSION_LESS "${JSONCPP_NEWEST_VALIDATED_POLICIES_VERSION}")
     #Set and use the newest available cmake policies that are validated to work
     set(JSONCPP_CMAKE_POLICY_VERSION "${CMAKE_VERSION}")
 else()