ignore yanked warden gem

This is a minor fix.

Warden 1.2.2 was yanked from Gemcutter, I've just added an extra restriction `"!= 1.2.2"` to ignore it.

I'm not sure how useful this would be since any new devise 2 users would get the newer (1.2.3) warden version, but as we have an app at production already we've got some weird bundler errors.

```
Could not find warden-1.2.2 in any of the sources
```

Cheers.