Updated jQuery, specified version of RDiscount

Hi Jeff,

I came across the markdown preview gem and realized it would be a pretty convenient solution. There were a couple problems I discovered, some jQuery selectors were out of date, and it failed when working with the latest version of RDiscount. To get it working, I just specified a two-year old version of RDiscount in my gemfile. Ideally, getting it to work with the latest version would be the way to go.