RSpec example fails after updating rspec and ammeter dependencies

I'm working on Rails 4 support for this library, and I've hit a small issue with a failing test case. I've tracked the problem back to the same test case failing on the master branch if the dependencies are updated.

I'm happy to continue investigating this, but I can't understand why the assertions in this test case are valid. I was hoping someone could shed some light on it for me.

The failure is [many_reference_proxy_spec.rb:145](https://github.com/basho/ripple/blob/master/spec/ripple/associations/many_reference_proxy_spec.rb#L145):

      1) Ripple::Associations::ManyReferenceProxy#keys maintains the list of keys properly as new documents are appended
         Failure/Error: @account.payment_methods.should have(1).key
           expected 1 key, got 3
          # ./spec/ripple/associations/many_reference_proxy_spec.rb:148:in `block (3 levels) in <top (required)>'

As far as I can see, the `@keys` collection in [ManyReferenceProxy](https://github.com/basho/ripple/blob/master/lib/ripple/associations/many_reference_proxy.rb) doesn't get updated when `<<` is called, so there will always be 3 keys (1, 2 and 3 as returned by the mock defined at line 120).

Anyone able to offer some advice?