use rubyracer for faster prefixing

Gemfile

@@ -14,6 +14,8 @@ gem 'w3c_validators'
 # ical files
 gem 'icalendar'
 
+gem 'therubyracer'
+
 # Autoprefixing for class
 gem 'autoprefixer-rails'
 

Gemfile.lock

@@ -38,6 +38,7 @@ GEM
     icalendar (2.4.1)
     json (2.0.2)
     kramdown (1.12.0)
+    libv8 (3.16.14.15)
     listen (3.1.5)
       rb-fsevent (~> 0.9, >= 0.9.4)
       rb-inotify (~> 0.9, >= 0.9.7)
@@ -71,6 +72,9 @@ GEM
     shellany (0.0.1)
     slop (3.6.0)
     terminal-notifier-guard (1.7.0)
+    therubyracer (0.12.2)
+      libv8 (~> 3.16.14.0)
+      ref
     thor (0.19.1)
     w3c_validators (1.2)
       json
@@ -91,6 +95,7 @@ DEPENDENCIES
   nanoc (= 4.3.7)
   sass
   terminal-notifier-guard
+  therubyracer
   w3c_validators
 
 BUNDLED WITH

lib/filters/autoprefixer.rb

@@ -1,4 +1,6 @@
+require 'v8'
 require 'autoprefixer-rails'
+
 Nanoc::Filter.define(:autoprefixer) do |content, _params|
   AutoprefixerRails.process(content).css
 end