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.

The Polycom support website should be consulted for further information
on upgrading.

The updater is what was previously known as "BootROM".

Needed for IP6000 and IP7000.

Default is 300.

Also refactor a little.

Superseded by 1.3.2.0.

Also link the version between the different xivo-yealink plugins since
they have lots of thing in common.

Ticket #4364.