git SHA should be resolved in lockfile

Given a `Berksfile`:

```ruby
cookbook 'cookbook', github: 'my-cbs/cookbook', branch: 'master'
```

The `Berskfile.lock` contains:

```json
{
  "sha":"b67b7b50e5aea1593a19f641453d6eeb46067e67",
  "sources":{
    "cookbook":{
      "locked_version":"1.0.0",
      "git":"git://github.com/my-cbs/cookbook",
      "ref":"master"
    }
  }
}
```

But it should really contain:

```json
{
  "sha":"b67b7b50e5aea1593a19f641453d6eeb46067e67",
  "sources":{
    "cookbook":{
      "locked_version":"1.0.0",
      "git":"git://github.com/my-cbs/cookbook",
      "ref":"abcde12345"
    }
  }
}
```

So my team can always pull the same version.