Suggested change:

``` ruby
RSpec.deprecate("`mock_model`", :replacement => "the `rspec-activemodel-mocks` gem")
```

* Elsewhere our deprecations are just in the form "`method_name`" rather than "the method_name method".
* The `starting in RSpec 3` part feels unnecessary.

BTW, does your `rspec-activemodel-mocks` gem work on RSpec 2.99?  If not, it's important to get it to work.  The 2.14 => 2.99 => 3.0 migration path is that users upgrade to 2.99, get warning free, then upgrade to 3.0, but if your gem doesn't work on 2.99 there's no way for the user to get warning free on 2.99.  When your gem is loaded on 2.99 it should work and silence this warning. (e.g. by causing your gem's `mock_model` to be called instead of this one).