Fix minify generation of ./_site/ (#863)

_plugins/minify-assets.rb

@@ -1,5 +1,8 @@
 # Minify assets after each build
 Jekyll::Hooks.register :site, :post_write do
   puts "Minifying assets"
-  `minify -r -o _site/ _site/`
+  # Project path
+  site_path_from = File.join(Dir.pwd, "_site")
+  site_path_to = Dir.pwd
+  `minify -r -o #{site_path_to} #{site_path_from}`
 end