Revert "Bump kramdown from 2.1.0 to 2.3.0 in /docs (#214)"

This reverts commit a552470.

Author: Flavio Corpa

docs/Gemfile.lock

@@ -33,8 +33,7 @@ GEM
       sassc (> 2.0.1, < 3.0)
     jekyll-watch (2.2.1)
       listen (~> 3.0)
-    kramdown (2.3.0)
-      rexml
+    kramdown (2.1.0)
     kramdown-parser-gfm (1.1.0)
       kramdown (~> 2.0)
     liquid (4.0.3)
@@ -48,7 +47,6 @@ GEM
     rb-fsevent (0.10.3)
     rb-inotify (0.10.0)
       ffi (~> 1.0)
-    rexml (3.2.4)
     rouge (3.14.0)
     safe_yaml (1.0.5)
     sassc (2.2.1)