facebook connection with oauth2 gem. Works under 1.9.2 and error in ruby 1.8.7

facebook.rb

require 'rubygems'

# require 'faraday'
# module Faraday 
  # class Adapter < Middleware
    # def call(env)
      # env[:ssl][:verify] = false if env[:ssl]
      # process_body_for_request(env)
    # end
  # end
# end

require 'sinatra'
require 'oauth2'
require 'json'

set :port, 80

def client
  OAuth2::Client.new('app_id', 'app_secret', :site => 'https://graph.facebook.com')
end

get '/auth/facebook' do
  redirect client.web_server.authorize_url(
    :redirect_uri => redirect_uri,
    :scope => 'email,offline_access'
  )
end

get '/facebook/auth/callback' do
  access_token = client.web_server.get_access_token(params[:code], :redirect_uri => redirect_uri)
  user = JSON.parse(access_token.get('/me'))

  user.inspect
end

def redirect_uri
  "your url here"
end