Update Jekyll to v3.5.1.

Gemfile

 source 'https://rubygems.org'
 
 group :development, :test do
-  gem 'jekyll', '~> 3.4.5'
+  gem 'jekyll', '~> 3.5.1'
   gem 'jekyll-redirect-from', '~> 0.12.1'
   gem 'jekyll-sitemap', '~> 1.1.1'
   gem 'jekyll-toc', '~> 0.3.0.pre1'

Gemfile.lock

@@ -7,13 +7,13 @@ GEM
     ffi (1.9.18)
     ffi (1.9.18-x64-mingw32)
     forwardable-extended (2.6.0)
-    jekyll (3.4.5)
+    jekyll (3.5.1)
       addressable (~> 2.4)
       colorator (~> 1.0)
       jekyll-sass-converter (~> 1.0)
       jekyll-watch (~> 1.1)
       kramdown (~> 1.3)
-      liquid (~> 3.0)
+      liquid (~> 4.0)
       mercenary (~> 0.3.3)
       pathutil (~> 0.9)
       rouge (~> 1.7)
@@ -24,12 +24,12 @@ GEM
       sass (~> 3.4)
     jekyll-sitemap (1.1.1)
       jekyll (~> 3.3)
-    jekyll-toc (0.3.0.pre1)
+    jekyll-toc (0.3.0)
       nokogiri (~> 1.6)
     jekyll-watch (1.5.0)
       listen (~> 3.0, < 3.1)
     kramdown (1.14.0)
-    liquid (3.0.6)
+    liquid (4.0.0)
     listen (3.0.8)
       rb-fsevent (~> 0.9, >= 0.9.4)
       rb-inotify (~> 0.9, >= 0.9.7)
@@ -58,11 +58,11 @@ PLATFORMS
   x64-mingw32
 
 DEPENDENCIES
-  jekyll (~> 3.4.5)
+  jekyll (~> 3.5.1)
   jekyll-redirect-from (~> 0.12.1)
   jekyll-sitemap (~> 1.1.1)
   jekyll-toc (~> 0.3.0.pre1)
   scss_lint (~> 0.54.0)
 
 BUNDLED WITH
-   1.15.1
+   1.15.3

_config.yml

@@ -17,7 +17,7 @@ url:              https://getbootstrap.com
 encoding:         UTF-8
 exclude:          [.git, .github, assets/scss/, build, js, node_modules, nuget, scss, vendor, bower.json, composer.json, Gemfile, Gemfile.lock, Gruntfile.js, package.js, package.json, package-lock.json, sache.json]
 
-gems:
+plugins:
   - jekyll-redirect-from
   - jekyll-sitemap
   - jekyll-toc