Add sass requirement
Add sass requirement

Neat adds it's stylesheets to the Sass load paths however we are not
requiring sass explicitly. Previously, Sass was being required
implicitly through bourbon however, as of bourbon-5.0 Sass is no longer
required in the gem itself resulting in an error within Neat.

Resolves
https://github.com/thoughtbot/neat/issues/429

craftsmen/neat,thoughtbot/neat,thoughtbot/neat,craftsmen/neat,craftsmen/neat,thoughtbot/neat