Gemfile: add kramdown-parser-gfm

Jekyll failed building while parsing markdown. Adding this module made
it work again. Feel free to revert if you know why it broke in first
place.

Gemfile

@@ -19,6 +19,7 @@ group :jekyll_plugins do
   gem 'jekyll-twitter-plugin'
   gem "tzinfo-data"
   gem "tzinfo"
+  gem "kramdown-parser-gfm"
 end
 
 require 'rbconfig'