Default :sleep value must be an instance of Period

Default sleep value can't use (clj-time.core/secs 1) since that returns an instance of Second (even though the documentation says otherwise) not Period. Causes a method not found .toStandardSeconds in  period->secs.

Trying to verify if this is a bug in clj-time, but not likely that they will change it since it's going to break existing clients.

https://github.com/clj-time/clj-time/blob/master/src/clj_time/core.clj#L354