Bump kramdown from 1.17.0 to 2.4.0 (#2627)

Bumps [kramdown](https://github.com/gettalong/kramdown) from 1.17.0 to 2.4.0.
- [Release notes](https://github.com/gettalong/kramdown/releases)
- [Changelog](https://github.com/gettalong/kramdown/blob/master/doc/news.page)
- [Commits](https://github.com/gettalong/kramdown/commits)

---
updated-dependencies:
- dependency-name: kramdown
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Gemfile

@@ -19,7 +19,7 @@ gem 'pry-rescue'
 # Optional middleman dependencies, included for tests
 gem 'coffee-script', '~> 2.2', require: false
 gem 'haml', '~> 4.0', require: false
-gem 'kramdown', '~> 1.2', require: false
+gem 'kramdown', '~> 2.4', require: false
 gem 'liquid', '~> 3.0', require: false
 gem 'redcarpet', '>= 3.1', require: false
 gem 'sinatra', '>= 2.0.0.beta2', require: false

Gemfile.lock

@@ -100,7 +100,8 @@ GEM
     i18n (1.6.0)
       concurrent-ruby (~> 1.0)
     interception (0.5)
-    kramdown (1.17.0)
+    kramdown (2.4.0)
+      rexml
     liquid (3.0.6)
     listen (3.0.8)
       rb-fsevent (~> 0.9, >= 0.9.4)
@@ -225,7 +226,7 @@ DEPENDENCIES
   coffee-script (~> 2.2)
   cucumber (~> 2.0)
   haml (~> 4.0)
-  kramdown (~> 1.2)
+  kramdown (~> 2.4)
   liquid (~> 3.0)
   middleman-cli!
   middleman-core!