Adjust the Python http_pipe.py script to work with our default version of
Python.

This change will be sent back up stream for consideration.

--- tests/http_pipe.py.orig	2014-09-10 15:59:26.498743943 -0700
+++ tests/http_pipe.py	2014-09-10 15:58:59.502589711 -0700
@@ -435,7 +435,7 @@
 if args.pidfile:
     pid = os.getpid()
     f = open(args.pidfile, 'w')
-    f.write('{}'.format(pid))
+    f.write('{0}'.format(pid))
     f.close()
 
 server = PipelineServer(('0.0.0.0', args.port), PipelineRequestHandler)