update ngrok

@iElectric Here you go :)

Generated via:

``` bash
go2nix --until auto --rev b7d5571aa7f12ac304b8f8286b855cc64dd9bab8 --tags release --pkg github.com/inconshreveable/ngrok --out-nix deps.nix
```

**Question:** The `go-bindata` tool, as I understand it, takes the assets and generates go code embedding the resources. Is there any reason to still invoke `cp -R assets $out`? @iElectric, could you perhaps test this without that line? (I've never used ngrok...)