Correct argument order by using keyword for keepalive.

Fixes a minor bug which causes the automatic pinging (from XEP_0199.enable_keepalive) to fail with a harmless error. Nothing bad happens, except we aren't actually pinging the other side.

Commit 3423589b changed the ordering of the arguments such that ifrom is the second argument in ping().

However keepalive is still passing timeout as the second argument. This causes all pings to fail with the following exception:

    Traceback (most recent call last):
    File "build/bdist.linux-x86_64/egg/sleekxmpp/plugins/xep_0199/ping.py", line 109, in _keepalive
      rtt = self.ping(self.xmpp.boundjid.host, self.timeout)
    File "build/bdist.linux-x86_64/egg/sleekxmpp/plugins/xep_0199/ping.py", line 174, in ping
      self.send_ping(jid, ifrom=ifrom, timeout=timeout)
    File "build/bdist.linux-x86_64/egg/sleekxmpp/plugins/xep_0199/ping.py", line 146, in send_ping
      return iq.send(block=block, timeout=timeout, callback=callback)
    File "build/bdist.linux-x86_64/egg/sleekxmpp/stanza/iq.py", line 227, in send
      StanzaBase.send(self, now=now)
    File "build/bdist.linux-x86_64/egg/sleekxmpp/xmlstream/stanzabase.py", line 1595, in send
      self.stream.send(self, now=now)
    File "build/bdist.linux-x86_64/egg/sleekxmpp/xmlstream/xmlstream.py", line 1209, in send
      top_level=True)
    File "build/bdist.linux-x86_64/egg/sleekxmpp/xmlstream/tostring.py", line 89, in tostring
      value = escape(value, use_cdata)
    File "build/bdist.linux-x86_64/egg/sleekxmpp/xmlstream/tostring.py", line 149, in escape
      text = unicode(text, 'utf-8', 'ignore')
    TypeError: coercing to Unicode: need string or buffer, int found