[#146] Fix executor type ambiguity with Clojure 1.6?

Having difficulty reproducing the error described by the GitHub issue.

Figure the cause might have something to do with attaching the type info
to the (fn []) form? The approach as of this commit seems like it'd be
less likely to trip up the compiler.

src/taoensso/timbre.cljx

@@ -565,14 +565,14 @@
       ;; that starts the Clojure agent threadpool which can slow application
       ;; shutdown w/o a `(shutdown-agents)` call
       (let [executor (java.util.concurrent.Executors/newSingleThreadExecutor)
-            f_ (.submit executor ^java.util.concurrent.Callable
+            ^java.util.concurrent.Callable f
             (fn []
               (try
                 (.. java.net.InetAddress getLocalHost getHostName)
                 (catch java.net.UnknownHostException _ "UnknownHost")
-                     (finally (.shutdown executor)))))]
+                (finally (.shutdown executor))))]
 
-        (deref f_ 5000 "UnknownHost")))))
+        (deref (.submit executor f) 5000 "UnknownHost")))))
 
 (comment (get-hostname))