update autoprefixer gem

Guardfile

@@ -70,15 +70,15 @@ guard :minitest, spring: "bin/rails test", all_after_pass: true do
   watch(%r{^app/views/(.*_mailer/)?([^/]+)\.erb$}) { ["test/mailers", "test/integration"] }
 end
 
-# ESLint
 guard :shell, all_on_start: true do
-  # TODO: Annoyingly, all files are linted twice on start/full runs. Why?
+  # ESLint
   watch %r{app/assets/javascripts/*/.*} do |file|
     system %(echo "ESLint:\033[32m #{file[0]}\033[0m")
     system %(./node_modules/eslint/bin/eslint.js #{file[0]})
   end
 
-  watch %r{app/assets/stylesheets/*/.*} do |file|
+  # sass-lint
+  watch %r{app/assets/stylesheets/*/.*scss$} do |file|
     system %(echo "sass-lint:\033[32m #{file[0]}\033[0m")
     system %(sass-lint --cache --config .sass-lint.yml '#{file[0]}' --verbose --no-exit)
   end