update jekyll config

The "gem" configuration option is marked as obsolete and has been renamed
to "plugins".

Author: Johann150

_config.yml

@@ -23,7 +23,7 @@ url: "https://fablab-cottbus.de"
 highlighter: rouge
 markdown: kramdown
 theme: minima
-gems:
+plugins:
   - jekyll-feed
   - jekyll-bootstrap-sass
   - jekyll-lunr-js-search