Use the GitHub Pages Gem

The GitHub Pages Gem (https://github.com/github/pages-gem)
is a meta-gem that does two things:

1. It locks dependencies to the same version used by GitHub Pages
ensuring that when you build the site locally, you're using the same
version of plugins and other dependencies used in production.

2. When loaded as part of the :jekyll_plugins group, it allows
the Gem to set certain configuration defaults and overrides
(such as activating default plugins) to ensure, once again
that your local preview replicates the production version
as closely as possible.

Gemfile

@@ -1,20 +1,3 @@
 source "https://rubygems.org"
 
-gem "jekyll", "3.2.1"
-gem "jekyll-sass-converter", "1.3.0"
-gem "minima", "1.1.0"
-gem "kramdown", "1.11.1"
-gem "liquid", "3.0.6"
-gem "rouge", "1.11.1"
-gem "jemoji", "0.7.0"
-gem "jekyll-mentions", "1.2.0"
-gem "jekyll-redirect-from", "0.11.0"
-gem "jekyll-sitemap", "0.10.0"
-gem "jekyll-feed", "0.5.1"
-gem "jekyll-gist", "1.4.0"
-gem "jekyll-paginate", "1.1.0"
-gem "jekyll-coffeescript", "1.0.1"
-gem "jekyll-seo-tag", "2.0.0"
-gem "jekyll-github-metadata", "2.0.2"
-gem "listen", "3.0.6"
-gem "activesupport", "4.2.7"
+gem "github-pages", group: :jekyll_plugins