Update debian/watch to check upstream signature

Closes: #764768
Imported Upstream version 2.10.10
Merge tag 'upstream/2.10.10'

Upstream version 2.10.10
changelog
Import 2.10.9-0ubuntu7.1
Build 2.10.10 for the Pidgin PPA

Merge branch 'master' into pidgin-ppa/utopic

Conflicts:
	debian/control
	debian/libpurple0.symbols
Build 2.10.10 for Trusty