Increment version to 0.5.13 in Cmake

As per the newest release.

Author: sum01

CMakeLists.txt

@@ -1,5 +1,5 @@
 cmake_minimum_required(VERSION 3.7.0 FATAL_ERROR)
-project(httplib VERSION 0.5.12 LANGUAGES CXX)
+project(httplib VERSION 0.5.13 LANGUAGES CXX)
 
 # Change as needed to set an OpenSSL minimum version.
 # This is used in the installed Cmake config file.