webkitgtk: 2.4.6 -> 2.6.1

Moved from #4433 .
2.6.1 is indeed v2, however it is not compatible with 2.4.6, cf https://github.com/NixOS/nixpkgs/pull/4433#issuecomment-59173688.
We could set `webkitgtk2` to point to 2.6.\*, but then all reverse dependencies should be updated to explicitly depend on 2.4.\* . What do you prefer ?