(SERVER-150) flush jruby instances

I think it is possible to simplify this test by simply removing both the promise and the watch ...
```diff
diff --git a/test/unit/puppetlabs/services/jruby/jruby_puppet_agents_test.clj b/test/unit/puppetlabs/services/jruby/jruby_puppet_agents_test.clj
index 2de1591..ba6411a 100644
--- a/test/unit/puppetlabs/services/jruby/jruby_puppet_agents_test.clj
+++ b/test/unit/puppetlabs/services/jruby/jruby_puppet_agents_test.clj
@@ -75,18 +75,10 @@
       (let [jruby-service (tk-app/get-service app :JRubyPuppetService)
             context (tk-services/service-context jruby-service)
             pool-context (:pool-context context)
-            old-pool (jruby-core/get-pool pool-context)
-            pool-state-swapped (promise)
-            pool-state-watch-fn (fn [key pool-state old-val new-val]
-                                  (when (not= (:pool old-val) (:pool new-val))
-                                    (remove-watch pool-state key)
-                                    (deliver pool-state-swapped true)))]
+            old-pool (jruby-core/get-pool pool-context)]
         ; borrow an instance so we know that the pool is ready
         (jruby/with-jruby-puppet jruby-puppet jruby-service)
-        (add-watch (:pool-state pool-context) :pool-state-watch pool-state-watch-fn)
         (jruby-protocol/flush-jruby-pool! jruby-service)
-        ; wait until we know the new pool has been swapped in
-        @pool-state-swapped
         ; wait until the flush is complete
         (await (:pool-agent context))
         (let [old-pool-instance (jruby-core/borrow-from-pool old-pool)]
```

Basically, I think `await` takes care of everything for you, and I think the promise + watch are essentially redundant.  But I could be wrong ...