Add json gem for ruby 1.8 users

Ran into this while setting up a fresh jekyll env:
http://stackoverflow.com/questions/19788258/error-when-running-jekyll-new-command
https://github.com/jekyll/jekyll/issues/1727

Ruby 1.8 users need to install the json gem manually.
Merge pull request #3 from liliakai/json-error

Add note about json dependency to README