Bump kramdown from 2.3.0 to 2.3.1 (ReactiveX#2304)

Author: dependabot[bot]

Gemfile.lock

@@ -89,7 +89,7 @@ GEM
     i18n (0.9.5)
       concurrent-ruby (~> 1.0)
     json (2.3.1)
-    kramdown (2.3.0)
+    kramdown (2.3.1)
       rexml
     kramdown-parser-gfm (1.1.0)
       kramdown (~> 2.0)