Merge pull request #62 from timku/patch-1

Update xkcd.lua fixed issue with random xkcd.

plugins/xkcd.lua

@@ -20,7 +20,7 @@ end
 function get_xkcd_random()
   local last = get_last_id()
   math.randomseed(os.time())
-  i = math.random(1, data.num)
+  i = math.random(1, last)
   return get_xkcd(i)
 end
 
@@ -51,4 +51,4 @@ return {
       "xkcd.com/(%d+)"
     }, 
     run = run 
-}
+}