Allow Importer to be serialized

This fixes the infamous `can't dump anonymous class` errors while attempting to save sass cache files (see #242).

This is not an issue for pre v5 versions as they do not support sass > 3.2

The problem comes from the sprockets context being an attribute of the importer class and thus making it impossible to serialize it (as far as I understand this).

Inspired by the `sprockets-sass` gem [Importer class](https://github.com/petebrowne/sprockets-sass/blob/master/lib/sprockets/sass/importer.rb).