docs: fix outdated CMake version requirement in README (#990)

The README stated that CMake 2.8.5+ was required, but CMakeLists.txt
requires CMake 3.5+. This inconsistency caused confusion for users
with CMake versions between 2.8.5 and 3.5.

Also updated library_config/uninstall.cmake to match for consistency.

Fixes #988

Author: PeterAlfredLee

README.md

@@ -89,7 +89,7 @@ cJSON is written in ANSI C (C89) in order to support as many platforms and compi
 
 #### CMake
 
-With CMake, cJSON supports a full blown build system. This way you get the most features. CMake with an equal or higher version than 2.8.5 is supported. With CMake it is recommended to do an out of tree build, meaning the compiled files are put in a directory separate from the source files. So in order to build cJSON with CMake on a Unix platform, make a `build` directory and run CMake inside it.
+With CMake, cJSON supports a full blown build system. This way you get the most features. CMake with an equal or higher version than 3.5 is supported. With CMake it is recommended to do an out of tree build, meaning the compiled files are put in a directory separate from the source files. So in order to build cJSON with CMake on a Unix platform, make a `build` directory and run CMake inside it.
 
 ```
 mkdir build

library_config/uninstall.cmake

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 2.8.5)
+cmake_minimum_required(VERSION 3.5)
 
 set(MANIFEST "${CMAKE_CURRENT_BINARY_DIR}/install_manifest.txt")