Upgrade slixmpp to 1.4.2

[fabaff]

Description:

Commit log: https://lab.louiz.org/poezio/slixmpp/commits/master

Example entry for configuration.yaml (if applicable):

notify:
  - platform: xmpp
    name: jabber
    sender: !secret xmpp_sender
    password: !secret xmpp_password
    recipient: !secret xmpp_recipient

{
  "message": "The sun is {% if is_state('sun.sun', 'above_horizon') %}up{% else %}down{% endif %}!"
}

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.

If the code communicates with devices, web services, or third-party tools:

• New dependencies have been added to the REQUIREMENTS variable ([example][ex-requir]).
• New or updated dependencies have been added to requirements_all.txt by running script/gen_requirements_all.py.