Read Sinatra's views path via :view_path option

Currently Sinatra's views path is fetched via `scope` variable, but it can be non-Sinatra-application instance when `:scope` option is passed like below:

```ruby
jbuilder :some_template, :scope => Object.new
```

So I wrapped `Sinatra::Template#render` to pass Sinatra's views path (including `:views` option) by copying it into `:view_path` option, and removed codes dependent on Sinatra from `Tilt::Jbuilder`.