Avoid subclassing Sprockets::SassTemplate

Some of these internal methods have shifted around. Sprockets 3.x doesn't even use Tilt as a base anymore. Just copied in the few superclass methods we were relying on. Should be pretty independent to function on 2.x and 3.x

/cc @rafaelfranca 
I think we can remove this file. I remember I added this same function on sprockets https://github.com/sstephenson/sprockets/blob/master/lib/sprockets/sass_template.rb#L223