Fixes #366

Fallback for pre rails 4 sprockets (< 2.10.0)
Use correct reference to context through the importer

Fixes #366 in the correct way, hopefully.

I can't cite my sources well, since Sprockets is very difficult to follow... but suffice to say the global options hash sent to the Tilt SASS template engine always has a reference to the Sprockets importer, and the importer always has a reference to the context. Tested with Sprockets 2.2.2 with and without Rails.
Merge pull request #373 from trisweb/trisweb_glebm_fix_sprockets_context

Fix the Sprockets context reference for pre-2.10 Sprockets