exposing context to sass compressor fixing function calls

sass_functions.rb references the context from options[sprockets][context]

https://github.com/sstephenson/sprockets/blob/v2.10.0/lib/sprockets/sass_functions.rb#L63

This blows up when you try to run code that calls these helper functions through the sass compressor because it instantiates the sass engine without these options.  This doesn't happen when running without the compressor because the Sass Template provides this option already.

https://github.com/sstephenson/sprockets/blob/v2.10.0/lib/sprockets/sass_template.rb