Bump kramdown from 2.1.0 to 2.3.1

Bumps [kramdown](https://github.com/gettalong/kramdown) from 2.1.0 to 2.3.1.
- [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)

Signed-off-by: dependabot[bot] <support@github.com>

Gemfile.lock

@@ -93,7 +93,8 @@ GEM
       sqlite3 (~> 1.3)
       xcinvoke (~> 0.3.0)
     json (2.3.0)
-    kramdown (2.1.0)
+    kramdown (2.3.1)
+      rexml
     kramdown-parser-gfm (1.1.0)
       kramdown (~> 2.0)
     liferaft (0.0.6)
@@ -111,6 +112,7 @@ GEM
     open4 (1.3.4)
     public_suffix (4.0.3)
     redcarpet (3.5.0)
+    rexml (3.2.4)
     rouge (3.15.0)
     ruby-macho (1.4.0)
     sassc (2.2.1)
@@ -141,4 +143,3 @@ DEPENDENCIES
   cocoapods
   danger
   jazzy
-