Bump minimum cmake version (#323)

Avoids the annoying warning messages such as:

```
CMake Deprecation Warning at CMakeLists.txt:1 (cmake_minimum_required):
  Compatibility with CMake < 3.10 will be removed from a future version of
  CMake.

  Update the VERSION argument <min> value.  Or, use the <min>...<max> syntax
  to tell CMake that the project requires at least <min> but has been updated
  to work with policies introduced by <max> or earlier.
```
and allows to get rid of the policy definition

CMakeLists.txt

@@ -1,9 +1,4 @@
-cmake_minimum_required(VERSION 3.5)
-
-# See https://cmake.org/cmake/help/latest/policy/CMP0077.html
-# This allows for setting option variables externally, when this project
-# is included in another CMake project.
-cmake_policy(SET CMP0077 NEW)
+cmake_minimum_required(VERSION 3.13)
 
 project(inja
         DESCRIPTION "Template engine for modern C++"