Fix missing jekyll-gist gem in _config.yml

[indrakaw]

Without jekyll-gist, I get this error:

$ bundle exec jekyll s
Configuration file: /home/indrakaw/html/shiori/_config.yml
Configuration file: /home/indrakaw/html/shiori/_config.yml
            Source: /home/indrakaw/html/shiori
       Destination: /home/indrakaw/html/shiori/_site
 Incremental build: disabled. Enable with --incremental
      Generating... 
  Liquid Exception: Liquid syntax error (line 58): Unknown tag 'gist' in /home/indrakaw/html/shiori/_posts/2014-0

[rlucca]

Thanks.