Respect custom root path

[JaycobA]

• sourcemaps working (rails 4.1)
• respect relative_root_url setting
• inherit from Sass::Rails to enable globbing

[vhyza]

Hello @JaycobA. I'm terribly sorry for the delay.

Unfortunately it's not working for rails 4.2. It seems Rails.configuration.relative_url_root is nil in new rails 4.2 project.