Mongoid_slug is not installed

slug_error

I have gem mongoid (3.0.4, 3.0.1)

My Gemfile looks like:
----------------------
source 'https://rubygems.org'

gem 'rails', '3.2.6'

# Bundle edge Rails instead:
# gem 'rails', :git => 'git://github.com/rails/rails.git'

gem "mongoid", "~> 3.0.0"

# Gems used only for assets and not required
# in production environments by default.
group :assets do
  gem 'sass-rails',   '~> 3.2.3'
  gem 'coffee-rails', '~> 3.2.1'

  # See https://github.com/sstephenson/execjs#readme for more supported runtimes
  gem 'execjs'
  gem 'therubyracer', :platforms => :ruby

  gem 'uglifier', '>= 1.0.3'
end

gem 'jquery-rails'
gem 'stringex'
gem 'mongoid_slug'


Then,
$> bundle install
Fetching gem metadata from https://rubygems.org/.........
Fetching gem metadata from https://rubygems.org/..
Bundler could not find compatible versions for gem "mongoid":
  In Gemfile:
    mongoid_slug (>= 0) ruby depends on
      mongoid (~> 2.0.0.beta7) ruby

    mongoid (3.0.1)


My Controller is:
------------------

class Post
  include Mongoid::Document
  include Mongoid::Slug
  field :name, type: String
  field :title, type: String
  field :content, type: String
  validates :title,  :presence => true, :length => { :minimum => 10 }
  has_many :comments
  slug :name
end



On webpage, when I run, I get:
------------

Routing Error

uninitialized constant Mongoid::Slug
Try running rake routes for more information on available routes.