Add sass-rails dependency to gemspec.

Created by: gracewashere

Fixes #134 (closed) Fixes #141 (closed)

Problem:

Without sass-rails in the host app's Gemfile, users were getting an error:

File to import not found or unreadable: normalize-rails/normalize.

Solution:

Add sass-rails as an explicit dependency, so users don't need to specify it in their own Gemfile.

Also remove a redundant inline_svg require statement.