Flavor::Large: fix warning about Plack::App::File on Plack(>= 1.0017)

in "Large" flavor and Plack (>= 1.0017), following warning appears:

```WARNING: Automatically converting Plack::App::File instance to a PSGI code reference. If you see this warning for each request, you probably need to explicitly call to_app() i.e. Plack::App::File->new(...)->to_app in your PSGI file.```

so I added `->to_app()` to `Plack::App::File->new(...)`.