Fix a local variable reference in BucketRateLimit.when.

ReferenceError: age is not defined	snowflake.js:265:7
        BucketRateLimit.prototype.when	snowflake/proxy/build/snowflake.js:265:7
        ProxyPair.prototype.flush	snowflake/proxy/build/snowflake.js:558:63
        bind/<	snowflake/proxy/build/snowflake.js:10:56
        ProxyPair.prototype.onClientToRelayMessage	snowflake/proxy/build/snowflake.js:495:14
        bind/<	snowflake/proxy/build/snowflake.js:10:56

proxy/util.coffee

@@ -174,7 +174,7 @@ class BucketRateLimit
 
   # How many seconds in the future will the limit expire?
   when: ->
-    age()
+    @age()
     (@amount - @capacity) / (@capacity / @time)
 
   isLimited: ->