Since we are using falcon's internal API, pre-release of 2.0.0a1 broke us since
we are using an internal `create_http_method_map` method. With 2.0.0 major
release, falcon dropped that method.

However, falcon refactored above mentioned method in 1.4.0 to two separate
map_http_methods and set_default_responders. We just adapt to that and set the
minimum required version to 1.4.0.