Support Sprocket-sass gem so it works outside a Rails env.

Sprockets-sass (used by Sinatra and Middleman) stores its sprockets context in a different location than Rails. This PR checks for this case and fixes: https://github.com/middleman/middleman/issues/1265
Merge pull request #608 from tdreyno/patch-1

Support Sprocket-sass gem so it works outside a Rails env.