webpack: Use the correct port in Casper tests.

Previously we used 9994 unconditionally, whereas we should be using
9984 or 9994 depending whether it's being run manually or via the
Casper tests.

tools/run-dev.py

@@ -74,7 +74,7 @@ os.setpgrp()
 # Pass --nostatic because we configure static serving ourselves in
 # zulip/urls.py.
 cmds = [['./tools/compile-handlebars-templates', 'forever'],
-        ['./tools/webpack', 'watch'],
+        ['./tools/webpack', '--watch', '--port', str(webpack_port)],
         ['python', 'manage.py', 'rundjango'] +
           manage_args + ['localhost:%d' % (django_port,)],
         ['python', 'manage.py', 'runtornado'] +