Fix example Gemfile in README.md

Hey Mattt,

AFIncrementalStore and Rack::CoreData are an amazing combo, thanks for creating both of them!

I spent some time today figuring out how to create an application using Rack::CoreData directly instead of using the buildpack, and I found a few issues with the example Gemfile in the README that tripped me up a bit. 

1. There should be no `$ ` at the beginning of the line.  As far as I can tell, that's just a typo.
2. `source :rubygems` needs to appear at the top of the Gemfile otherwise Heroku doesn't know where to look for the gems.
3. `gem 'thin'` and `gem 'pg'` need to be included in the Gemfile. What's more, as far as I can tell, they need to be present when the Heroku app is created and is first pushed via `git push heroku master`, otherwise, Heroku does not set up the Postgres database or at least it does not set the config vars of `DATABASE_URL` and `HEROKU_POSTGRESQL_{COLOR}_URL`.  Comparing against the code in `/example`, it still took me quite a few test runs to figure out that the Gemfile needed to be correct from the start.

To someone more fluent in Ruby, Rack, and Heroku, these modifications would probably be obvious, but for the sake of iOS devs like me who will be stumbling through all of this, these changes might be helpful.

Larry