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 
Ah, saw you want to keep support to sprockets 2.x so we will have to keep it.