gyp: Revive old SVN version to build Chromium.

Works around regression from a305e685.

We're also marking it lowPrio to make sure nobody will accidentally
reference it using nix-env -i.

Until we have fixed #7402, we're going to build with the old gyp version
to prevent being affected by https://crbug.com/462153

.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>