Fix memory leak in pods

This was particularly evident when doing `boot watch test` and has been fixed thanks to Micha's
intuition of adding a clojure.core/shutdown-agents in boot.pod/destroy-pod.

Fixes adzerk-oss/boot-test#26

boot/pod/src/boot/pod.clj

@@ -824,6 +824,7 @@
   [pod]
   (when pod
     (.close pod)
+    (clojure.core/shutdown-agents)
     (.. pod getClassLoader close)))
 
 (defn pod-pool