Do not install eol 'sass'

[EasterPeanut]

Because 'sass' is replaced by 'sassc' (middleman/middleman@36326e4) got some issues with sprockets: middleman/middleman-sprockets#132. Temporary solution is to still install 'sass'.

Originally posted by @EasterPeanut in #53 (comment)

Rather not include sass, so whenever this is fixed in middleman, remove 'sass'

[EasterPeanut]

Tried to install gems:

gem "sassc", "~> 2.2.1"
gem "sprockets", "~> 4.0.0"

But whenever activating sprockets without sass installed, the css will break.

[EasterPeanut]

Better enable the external pipeline feature of middleman and make use of webpack for compiling site.css and site.js > https://rossta.net/blog/using-webpack-with-middleman.html. So I can drop sprockets and sass after (and move scoped JS files to the partial directories) and run hound to see if Unknown property grid-template-columns warning still persists.

Related #7