Ticket #4660.

The plugin associator doesn't need the version number since it's never
extracted for Cisco SCCP phones.

The cisco.conf configuration now depends on an option which is available
starting from XiVO 13.17.

They were already only partially integrated in the plugin, and there is
no support in xivo-libsccp.

This reverts commit 0ca88720.

This reverts commit a860d7c1.

This reverts commit 2d1f8e51.

Without the check, if you install the plugin on an old xivo, i.e. a xivo
without the 13.15 version of xivo-fetchfw, then the plugin will not be
loaded, and this will confuse users.