Set Liquid version explicitly to 2.2.2, fixes #243

Obviously Liquid 2.3.0 seems to cause problems, see:
http://www.yodi.me/blog/2011/10/24/github-custom-domain-with-octopress-in-ubuntu-oneiric-11-dot-10/
http://stackoverflow.com/questions/7801197/syntax-highlighting-with-pygments-is-failing-via-liquid-templates-string-error

Gemfile

@@ -12,6 +12,7 @@ group :development do
   gem 'rubypants'
   gem 'rb-fsevent'
   gem 'stringex'
+  gem 'liquid', '2.2.2'
 end
 
 gem 'sinatra', '1.2.6'

Gemfile.lock

@@ -55,6 +55,7 @@ DEPENDENCIES
   compass (>= 0.11)
   haml (>= 3.1)
   jekyll
+  liquid (= 2.2.2)
   pygments.rb
   rack
   rake