Update preamble_size.py for recent API changes.

preamble_size.py

@@ -10,15 +10,18 @@ import econtext.master
 import econtext.ssh
 import econtext.sudo
 
-with econtext.master.Broker() as broker:
-    router = econtext.core.Router(broker)
-    context = econtext.master.Context(router, 0)
-    stream = econtext.ssh.Stream(router, 0, context.key, hostname='foo')
-    print 'SSH command size: %s' % (len(' '.join(stream.get_boot_command())),)
-    print 'Preamble size: %s (%.2fKiB)' % (
-        len(stream.get_preamble()),
-        len(stream.get_preamble()) / 1024.0,
-    )
+broker = econtext.master.Broker()
+
+router = econtext.core.Router(broker)
+context = econtext.master.Context(router, 0)
+stream = econtext.ssh.Stream(router, 0, context.key, hostname='foo')
+broker.shutdown()
+
+print 'SSH command size: %s' % (len(' '.join(stream.get_boot_command())),)
+print 'Preamble size: %s (%.2fKiB)' % (
+    len(stream.get_preamble()),
+    len(stream.get_preamble()) / 1024.0,
+)
 
 for mod in (
         econtext.master,