Bump kramdown from 2.4.0 to 2.5.1 (#711)

Author: dependabot[bot]

Gemfile.lock

@@ -32,8 +32,8 @@ 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 (3.9.0)
       rb-fsevent (~> 0.10, >= 0.10.3)
       rb-inotify (~> 0.9, >= 0.9.10)