Update Gemfile

revert for GitHub Pages

Author: YJPL

Gemfile

@@ -10,5 +10,6 @@ gem "jekyll-sitemap"
 gem "jekyll-feed"
 gem "jekyll-archives"
 gem "jekyll-seo-tag"
-gem "jekyll_picture_tag", "2.0.4"
+gem "jekyll_picture_tag", "~> 2.0"
+gem "webrick", "~> 1.8.1"
 end