Setting log_level earlier to silence info level configuration output

Using the `--quiet` flag with the `build` command still outputs logging for the configuration file. Setting the `log_level` earlier silences this.

```
$ bundle exec jekyll build --quiet
Configuration file: /a/directory/hello-world/_config.yml
```

This output is coming from here:  [`configuration.rb:141`](https://github.com/jekyll/jekyll/blob/master/lib/jekyll/configuration.rb#L141).