All that is required to bootstrap a basic rails application for a test suite.

application.rb

# spec/dummy/config/application.rb
ENV['BUNDLE_GEMFILE'] = File.expand_path('../../../../Gemfile', __FILE__)

require 'rubygems'
require 'bundler'

Bundler.setup

$:.unshift File.expand_path('../../../../lib', __FILE__)

# for everything: require "rails/all"
require "action_controller/railtie"

Bundler.require

ApplicationController = Class.new(ActionController::Base)

require 'your_gem'

module Dummy
  class Application < ::Rails::Application
    config.cache_classes = true
    config.active_support.deprecation = :stderr
    config.secret_token = 'http://s3-ec.buzzfed.com/static/enhanced/webdr03/2013/5/25/8/anigif_enhanced-buzz-11857-1369483324-0.gif'
  end
end

Dummy::Application.initialize!

config.ru

# spec/dummy/config.ru
require File.expand_path('../config/application', __FILE__)
run Dummy::Application

See gist at revision: https://gist.github.com/adamhunter/5687248/c2e908f5249130c050af74db1c26dfa6a565c980 for Rakefile and script/rails (they'd be in your gem root).