_config.yml: Remove decommissioned rdiscount

[riouwa]

In May 2016, GitHub decommissioned rdiscount and force use of kramdown.

Closes #494

[jarrekk]

Hi, since we haven't seen this repository's owner for a very long time, I forked it to my repo and built this website with a new domain name:
http://jekylltheme.org.

Anyone's theme who submitted it before will still be seen at http://jekylltheme.org and welcome everyone to contribute this website if you have nice themes.

Later I will beautify this repository to make it better, an excellent website needs everyone's contribution/PR too!😊

[gynter]

Thanks for the PR! @riosatria are You sure that doesn't break the repo? Jekyllthemes doesn't use Github Pages feature, the page is built locally on the hosting server by scripts.

[riouwa]

I've done my research and see no downside by switching to kramdown.

One thing you should note though, I'm not seeing any Gemfile on your repo.
Since you are not using GitHub Pages, kramdown need to be installed manually.

[gynter]

Exactly, but since I don't have access to web server (only @mattvh has), then I have no clue, if kramdown is installed or not.

[mattvh]

Somehow I don't have a gemfile for the project. I'll fix that up when I get around to upgrading everything to a modern version of Jekyll. (See #395)