Merge pull request #233 from ruby-syntax-tree/dependabot/bundler/kramdown-2.5.1

Bump kramdown from 2.4.0 to 2.5.1

Author: github-actions[bot]

Gemfile.lock

@@ -1,12 +1,12 @@
 GEM
   remote: https://rubygems.org/
   specs:
-    kramdown (2.4.0)
-      rexml
+    kramdown (2.5.1)
+      rexml (>= 3.3.9)
     power_assert (2.0.3)
     prettier_print (1.2.1)
     rake (13.2.1)
-    rexml (3.2.5)
+    rexml (3.3.9)
     rouge (4.5.1)
     syntax_tree (6.2.0)
       prettier_print (>= 1.2.0)