[manual] send_message: Rename recipient/stream fields to 'to'.

This commit changes APIs and requires and update of all zephyr
mirroring bots to deploy properly.

(imported from commit 2672d2d07269379f7a865644aaeb6796d54183e1)

api/bots/humbug_trac.py

@@ -41,7 +41,7 @@ def trac_subject(ticket):
 def send_update(ticket, content):
     client.send_message({
             "type": "stream",
-            "stream": "trac",
+            "to": "trac",
             "content": content,
             "subject": trac_subject(ticket)
             })