Fix incompatibility with Roundcube 1.3+

[diditopher]

Fixes the following issue: When the plugin is used with Roundcube 1.3+, "Preferences" -> "User Interface" stops working. In logs/errors the following message appears:

[20-Jan-2018 09:42:20 Europe/Berlin] PHP Fatal error:  Uncaught Error: Call to undefined function Q() in /srv/www/roundcube/beta/plugins/converse/converse.php:272
Stack trace:
#0 /srv/www/roundcube/beta/program/lib/Roundcube/rcube_plugin_api.php(440): converse->preferences_list(Array)
#1 /srv/www/roundcube/beta/program/steps/settings/func.inc(1265): rcube_plugin_api->exec_hook('preferences_lis...', Array)
#2 /srv/www/roundcube/beta/program/steps/settings/edit_prefs.inc(28): rcmail_user_prefs('general')
#3 /srv/www/roundcube/beta/index.php(303): include_once('/srv/www/roundc...')
#4 {main}
  thrown in /srv/www/roundcube/beta/plugins/converse/converse.php on line 272

This fix should work in older versions, but I only tested with Roundcube 1.3.4

[diditopher]

I also just updated composer.json to point to https://github.com/devurandom/roundcube-converse.js-xmpp-plugin instead of the unmaintained https://github.com/priyadi/roundcube-converse.js-xmpp-plugin. This should probably also be updated in the roundcube plugin repository

[devurandom]

LGTM