heroku_style_baby.rb

require 'rubygems'
require 'sinatra'
require 'json'

class GitHubUpdater

  def self.update?(json)
    (JSON.parse(json)["ref"] == "refs/heads/master")
  end

  def self.git_pull
    `cd /project/directory && git checkout master && git pull origin master`
  end

end

set :port, 54321

post '/' do
  GitHubUpdater.git_pull if GitHubUpdater.update?(params[:payload])
end