Upgrade kramdown.The patch fix security vulnerability

Author: socketopp

Gemfile.lock

@@ -33,7 +33,7 @@ GEM
       sassc (> 2.0.1, < 3.0)
     jekyll-watch (2.2.1)
       listen (~> 3.0)
-    kramdown (2.2.1)
+    kramdown (2.3.0)
       rexml
     kramdown-parser-gfm (1.1.0)
       kramdown (~> 2.0)