Require correct cmake version everywhere

CMakeLists.txt

@@ -1,8 +1,4 @@
-if(WIN32 OR APPLE)
-	cmake_minimum_required(VERSION 2.8.12)
-else()
-	cmake_minimum_required(VERSION 2.8.11)
-endif()
+cmake_minimum_required(VERSION 2.8.12)
 
 project(obs-studio)