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

Author: arichiardi

boot/pod/src/boot/pod.clj

@@ -823,6 +823,7 @@
   "Closes open resources held by the pod, making the pod eligible for GC."
   [pod]
   (when pod
+    (with-eval-in pod (clojure.core/shutdown-agents))
     (.close pod)
     (.. pod getClassLoader close)))