rspec-mocks 'allow' returns undefined method

I'm using RSpec2 v2.13.1 and it seems that rspec-mocks (https://github.com/rspec/rspec-mocks) should be included in it. Certainly it's listed in my Gemfile.lock.

However, when I run my tests I get

     Failure/Error: allow(Notifier).to receive(:new_comment) { @decoy }
 NoMethodError:
   undefined method `allow' for #<RSpec::Core::ExampleGroup::Nested_1::Nested_1:0x007fc302aeca78>

Here's the test I'm trying to run:

require 'spec_helper'

describe CommentEvent do

  before(:each) do
    @event = FactoryGirl.build(:comment_event)
    @decoy = double('Resque::Mailer::MessageDecoy', :deliver => true)
    # allow(Notifier).to receive(:new_comment) { @decoy }
    # allow(Notifier).to receive(:welcome_email) { @decoy }
  end

  it "should have a comment for its object" do
    @event.object.should be_a(Comment)
  end

  describe "email notifications" do
    it "should be sent for a user who chooses to be notified" do
      allow(Notifier).to receive(:new_comment) { @decoy }
      allow(Notifier).to receive(:welcome_email) { @decoy }
      [...]
    end

The goal is to stub out the notifier and message decoys such that I can test whether my CommentEvent class is indeed invoking the former. I read on rspec-mocks documentation that stubbing is not supported in before(:all), but it doesn't work in before(:each) either. Help!

Thanks for any insights...

Notifier, according to its name, is a Constant.

You can't double a constant with either allow or double. Instead you need to use stub_const

# Make a mock of Notifier at first
stub_const Notifier, Class.new

# Then stub the methods of Notifier
stub(:Notifier, :new_comment => @decoy)

Edit: Fixed syntax error on stub() call