make-ring: add webkitgtk as dependency

We also run apt-get with --ignore-missing option so that it doesn't
fail in case a dependency is missing. We use this to require both
libwebkit2gtk-4.0-dev and libwebkit2gtk-3.0-dev for ubuntu since
14.04 only has the latter.

Reviewed-by: Alexandre Viau <alexandre@alexandreviau.net>
Change-Id: I0b7428cd4ad2c7e2a10634aff14fe14e6fccc253