Add sprockets-helpers to the list of dependencies.
Add sprockets-helpers to the list of dependencies.

The sprockets-sass README recommends using this gem, and adding it
magically fixes the relative_assets-related issue #32.

ryanmark/middleman-sprockets,williamcoates/middleman-sprockets,williamcoates/middleman-sprockets,middleman/middleman-sprockets,ryanmark/middleman-sprockets,middleman/middleman-sprockets,ryanmark/middleman-sprockets,middleman/middleman-sprockets,williamcoates/middleman-sprockets