Bump kramdown from 2.4.0 to 2.5.1

Bumps [kramdown](https://github.com/gettalong/kramdown) from 2.4.0 to 2.5.1.
- [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)

---
updated-dependencies:
- dependency-name: kramdown
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

Gemfile.lock

@@ -45,8 +45,8 @@ GEM
       concurrent-ruby (~> 1.1)
       sorted_set (~> 1.0)
     json_schema (0.21.0)
-    kramdown (2.4.0)
-      rexml
+    kramdown (2.5.1)
+      rexml (>= 3.3.9)
     listen (3.4.1)
       rb-fsevent (~> 0.10, >= 0.10.3)
       rb-inotify (~> 0.9, >= 0.9.10)
@@ -113,7 +113,7 @@ GEM
       ffi (~> 1.0)
     rbtree (0.4.6)
     redcarpet (3.6.0)
-    rexml (3.3.9)
+    rexml (3.4.1)
     rspec (3.12.0)
       rspec-core (~> 3.12.0)
       rspec-expectations (~> 3.12.0)