require Ruby 2.0 or later

README.md

@@ -16,6 +16,8 @@ Or install it yourself as:
 
     $ gem install with_timed_cache
 
+**Note**: Ruby version 2.0 or later is required.
+
 ## Usage
 
     with_timed_cache(key, max_age: 1.hour) do
@@ -27,4 +29,3 @@ Or install it yourself as:
 * `max_age`: how long to use the cached data, ex: `30.minutes`
 * `location`: the directory path in which to store cache files
 * `format`: cache data may be persisted as `:json`, `:yaml`, or the default of marshaling objects to strings will be used.
-