Bump minimum CMake version to 3.5 (#811)

Compatibility with CMake < 3.5 will be removed in future versions of
CMake, so this avoids the warning and the impending doom.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 2f11b67..558d208 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.0)
+cmake_minimum_required(VERSION 3.5)
 
 if(POLICY CMP0077)
     cmake_policy(SET CMP0077 NEW)