Added require json to readme examples

To assist newer ruby developers who might hit an error (shown below), I've added `require 'json'` to the code example in the readme file. 

Under Ruby 2.0.0p247 when attempting to use the example code, the following error is presented: 

    [1] pry(main)> Congress.legislators
    Faraday::Error::ParsingError: uninitialized constant JSON::Parser
    from /Users/tibbon/.rvm/rubies/ruby-2.0.0-p247/lib/ruby/2.0.0/json/common.rb:155:in `parse'

All tests still pass, and no changes to other code were made. Verified issue on several computers.