Use absolute path for view_path

Current Chanko doesn't work with `rails server -d`. (daemon mode)
In daemonized Rack::Server, `Dir.pwd` doesn't return `Rails.root` but always `"/"`.

(See: https://github.com/rack/rack/blob/master/lib/rack/server.rb#L322 )

So When I set `units_directory_path` as follows, then `"app/units"` is expanded as `"/app/units"` as absolute path, it causes Missing Partial error.

```ruby
Chanko::Config.units_directory_path = "app/units"
```

This PR fixes this issue.