upgrade to new markdown interpreter

[crucialfelix]

The page build completed successfully, but returned the following warning:

Your site is using Maruku, the default Markdown interpreter. Maruku is now obsolete and may cause builds to fail for sites with invalid Markdown or HTML. See https://help.github.com/articles/migrating-your-pages-site-from-maruku for more information on upgrading to a newer Markdown interpreter.

For information on troubleshooting Jekyll see:

https://help.github.com/articles/using-jekyll-with-pages#troubleshooting

If you have any questions please contact us at https://github.com/contact.

[lisongx]

seems that add markdown: kramdown in _config.yml is enough, and jekyll generate sites without error