meson: Upgrade GLib / Gio dependency to version 2.68.0
Bump GLib dependency to version 2.68.0 in meson.build. Also remove gio version number from the dependency list in HACKING.md.
Fixes #176 (closed).
Bump GLib dependency to version 2.68.0 in meson.build. Also remove gio version number from the dependency list in HACKING.md.
Fixes #176 (closed).