make-ring: remove webkit2gtk-3.0 from dependencies

Since ring does not compile on Ubuntu 14.04, there is no point in
keeping this dependency. This will also prevent the make-ring
script from failing on Ubuntu 16.10, where --ignore-missing no longer
seems to work to prevent apt from failing when we try to install
a non-existant package.

Change-Id: I21ae61786aa1019b8b644115ff2c1cab54c566fe
Reviewed-by: Guillaume Roguez <guillaume.roguez@savoirfairelinux.com>