Use pre-defined const value `ActiveAppsTrimInterval` intead of `1*time.Minute` 

in `NewActiveApps`

There is a const value [ActiveAppsTrimInterval] (https://github.com/cloudfoundry/gorouter/blob/master/stats/active_apps.go#L10) defined but no one use it. Actually, it should be used in [NewActiveApps](https://github.com/cloudfoundry/gorouter/blob/master/stats/active_apps.go#L95) to replace the magic number `1*time.Minute` there.