Replace WebKit 3.0 bindings with 4.0.

WebKitGTK+ is bumping their API version to 4.0 starting with 2.5.1.
These bindings haven't been part of a stable Vala release, so it should
be safe to just bump the version we distribute, too, instead of adding
4.0 alongside 3.0.
bindings: Update GIR-based bindings