Set :expires_in on throttle cache counters

Should reduce memcached evictions

lib/rack/attack/cache.rb

@@ -9,11 +9,13 @@ module Rack
       end
 
       def count(unprefixed_key, period)
-        key = "#{prefix}:#{Time.now.to_i/period}:#{unprefixed_key}"
-        result = store.increment(key, 1)
+        epoch_time = Time.now.to_i
+        expires_in = period - (epoch_time % period)
+        key = "#{prefix}:#{epoch_time/period}:#{unprefixed_key}"
+        result = store.increment(key, 1, :expires_in => expires_in)
         # NB: Some stores return nil when incrementing uninitialized values
         if result.nil?
-          store.write(key, 1)
+          store.write(key, 1, :expires_in => expires_in)
         end
         result || 1
       end

lib/rack/attack/version.rb

@@ -1,5 +1,5 @@
 module Rack
   module Attack
-    VERSION = '1.3.0'
+    VERSION = '1.3.1'
   end
 end