fix deploy

Author: abnerlee

Gemfile

@@ -19,6 +19,7 @@ gem "kramdown"
 # If you have any plugins, put them here!
 group :jekyll_plugins do
   gem "jekyll-feed", "0.12"
+  gem "jekyll-last-modified-at"
 end
 
 # Windows and JRuby does not include zoneinfo files, so bundle the tzinfo-data gem

_config.yml

@@ -9,11 +9,13 @@ highlighter: null
 plugins:
   - jekyll-redirect-from
   - jekyll-feed
+  - jekyll-last-modified-at
 
-# GitHub Pages supported plugins
+# Plugins used by our custom Jekyll build
 whitelist:
   - jekyll-redirect-from
   - jekyll-feed
+  - jekyll-last-modified-at
 jekyll-redirect-from: true
 
 markdown: kramdown