can work with rails4+mongoid3

otherwise will get exception:
lib/doorkeeper/models/mongoid3/access_grant.rb:13:in `<class:AccessGrant>': uninitialized constant Moped::BSON (NameError)
	from /Users/axu/.rvm/gems/ruby-2.1.0/bundler/gems/doorkeeper-16abb492d57a/lib/doorkeeper/models/mongoid3/access_grant.rb:5:in `<module:Doorkeeper>'
	from /Users/axu/.rvm/gems/ruby-2.1.0/bundler/gems/doorkeeper-16abb492d57a/lib/doorkeeper/models/mongoid3/access_grant.rb:4:in `<top (required)>'
	from /Users/axu/.rvm/gems/ruby-2.1.0/bundler/gems/doorkeeper-16abb492d57a/lib/doorkeeper/config.rb:19:in `enable_orm'
	from /Users/axu/.rvm/gems/ruby-2.1.0/bundler/gems/doorkeeper-16abb492d57a/lib/doorkeeper/config.rb:10:in `configure'
	from /Users/axu/prj/howtodo/code/htd.login/config/initializers/doorkeeper.rb:1:in `<top (required)>'
	from /Users/axu/.rvm/gems/ruby-2.1.0/gems/railties-4.0.2/lib/rails/engine.rb:609:in `block (2 levels) in <class:Engine>'
	from /Users/axu/.rvm/gems/ruby-2.1.0/gems/railties-4.0.2/lib/rails/engine.rb:608:in `each'
	from /Users/axu/.rvm/gems/ruby-2.1.0/gems/railties-4.0.2/lib/rails/engine.rb:608:in `block in <class:Engine>'
	from /Users/axu/.rvm/gems/ruby-2.1.0/gems/railties-4.0.2/lib/rails/initializable.rb:30:in `instance_exec'
	from /Users/axu/.rvm/gems/ruby-2.1.0/gems/railties-4.0.2/lib/rails/initializable.rb:30:in `run'
	from /Users/axu/.rvm/gems/ruby-2.1.0/gems/railties-4.0.2/lib/rails/initializable.rb:55:in `block in run_initializers'
	from /Users/axu/.rvm/rubies/ruby-2.1.0/lib/ruby/2.1.0/tsort.rb:226:in `block in tsort_each'
	from /Users/axu/.rvm/rubies/ruby-2.1.0/lib/ruby/2.1.0/tsort.rb:348:in `block (2 levels) in each_strongly_connected_component'
	from /Users/axu/.rvm/rubies/ruby-2.1.0/lib/ruby/2.1.0/tsort.rb:418:in `block (2 levels) in each_strongly_connected_component_from'
	from /Users/axu/.rvm/rubies/ruby-2.1.0/lib/ruby/2.1.0/tsort.rb:427:in `each_strongly_connected_component_from'
	from /Users/axu/.rvm/rubies/ruby-2.1.0/lib/ruby/2.1.0/tsort.rb:417:in `block in each_strongly_connected_component_from'
	from /Users/axu/.rvm/gems/ruby-2.1.0/gems/railties-4.0.2/lib/rails/initializable.rb:44:in `each'
	from /Users/axu/.rvm/gems/ruby-2.1.0/gems/railties-4.0.2/lib/rails/initializable.rb:44:in `tsort_each_child'