Update prosody to 0.9.4.

Fix for a DoS vulnerability, see
  https://www.debian.org/security/2014/dsa-2895

Changes in 0.9.4:
- Compression: Disallow compression on unauthenticated streams
- Core: Limit default read size and maximum stanza size
- Core: Enable SASL EXTERNAL by default for component s2s
- S2S: Warn if s2s_secure_auth and s2s_require_encryption have been
  set in conflicting ways
- S2S: Warn if no local network addresses were found, preventing
  successful s2s
- MUC: Fix traceback when a non-occupant tried to change an
  occupant's role
- MUC: API: Fire an event when temporary rooms are destroyed after
  the last person leaves
- Telnet: Fixed traceback when listing users
- Telnet: Apply normalization to JIDs in user management commands
- HTTP: Fix directory detection in file server on Windows
- Plugins: Fix paths on Windows
- MOTD: Don't strip blank lines from the message provided in the config
- prosodyctl: Better error reporting when generating certificates
- Makefile: Improve FreeBSD compatibility
- Multiple fixes to our migration tools, and support for importing MUCs
  from ejabberd

(fhajny)