Added :refer-clojure :exclude for promise for actors namespace

When referring to co.paralleluniverse.pulsar.actor I get following error message:

IllegalStateException promise already refers to: #'co.paralleluniverse.pulsar.core/promise in namespace: co.paralleluniverse.pulsar.actors  clojure.lang.Namespace.warnOrFailOnReplace (Namespace.java:88) and the loading fails.

Used namespace declaration is following:

```clojure
(ns ptest
  (:refer-clojure :exclude [promise])
  (:require [co.paralleluniverse.pulsar.core :refer :all] :reload
               [co.paralleluniverse.pulsar.actors :refer [a]] :reload))
```

The error message doesn't happen if only accessing pulsar.core.

After adding the (:refer-clojure :exclude [promise]) to actors namespace the problem doesn't appear anymore.

Probably this happens because actors namespace refer pulsar.core :all.