Problem with dev mix deps

I noticed a failure on get the `mix.deps.get` in development mode:

```
Cloning into 'deps/docs_ghpages'...
remote: Counting objects: 12, done.
remote: Compressing objects: 100% (9/9), done.
remote: Total 12 (delta 0), reused 12 (delta 0)
fatal: reference is not a tree: fb0681291259aa366ed1b00d481655ce1b7501c5
** (Mix) command `git checkout --quiet fb0681291259aa366ed1b00d481655ce1b7501c5` failed
```

Bumping the version of docs_ghpages fixed this.