Fix compilation for CMake versions 3.0.x where x >= 1

Apparently policy CMP0015 was added in CMake 3.1. With CMake 3.0.2,
the build was failing with

    CMake Error at CMakeLists.txt:18 (CMAKE_POLICY):
      Policy "CMP0051" is not known to this version of CMake.

This patch makes it work.

CMakeLists.txt

@@ -14,7 +14,7 @@
 PROJECT(libgit2 C)
 CMAKE_MINIMUM_REQUIRED(VERSION 2.8.11)
 CMAKE_POLICY(SET CMP0015 NEW)
-IF (CMAKE_VERSION VERSION_GREATER 3.0)
+IF (NOT CMAKE_VERSION VERSION_LESS 3.1)
 	CMAKE_POLICY(SET CMP0051 NEW)
 ENDIF()