Update _config.yml: gems to plugins (#6082)
Update _config.yml: gems to plugins (#6082)

Merge pull request 6082
chrisfinazzo/jekyll,getandpost/jekyll.github.io,ryanshaw/jekyll,tillgrallert/jekyll,alex-kovac/jekyll,x-way/jekyll,tomjohnson1492/jekyll,jaybe-jekyll/jekyll,jekyll/jekyll,jekyll/jekyll,gencer/jekyll,jaybe-jekyll/jekyll,tomjohnson1492/jekyll,Strangehill/jekyll,x-way/jekyll,alex-kovac/jekyll,chrisfinazzo/jekyll,alex-kovac/jekyll,yhironaka/yhironaka.github.io,jekyll/jekyll,Strangehill/jekyll,fulldecent/jekyll,chrisfinazzo/jekyll,jaybe-jekyll/jekyll,alex-kovac/jekyll,fulldecent/jekyll,tomjohnson1492/jekyll,x-way/jekyll,getandpost/jekyll.github.io,jekyll/jekyll,chrisfinazzo/jekyll,x-way/jekyll,alex-kovac/jekyll,ryanshaw/jekyll,gencer/jekyll,tillgrallert/jekyll,tomjohnson1492/jekyll,tillgrallert/jekyll,gencer/jekyll,Strangehill/jekyll,gencer/jekyll,tomjohnson1492/jekyll,fulldecent/jekyll,tillgrallert/jekyll,jaybe-jekyll/jekyll,ryanshaw/jekyll,gencer/jekyll,Strangehill/jekyll,ryanshaw/jekyll,getandpost/jekyll.github.io,fulldecent/jekyll,tillgrallert/jekyll,Strangehill/jekyll