Removes 3-0-stable and 3-1-stable from the build matrix

As far as I can tell, these versions will never have new releases.
require "active_support" is always needed

Per <https://github.com/rails/rails/pull/13605#issuecomment-31626172>,
always load the base "active_support" file before loading specific parts
of active_support.
Deprecate `mock_model` and `stub_model`

For rspec/rspec-rails#894
Use `expect_deprecation_with_call_site`

Added the function `expect_deprecation_with_call_site` to
support/helpers and updated the current deprecation specs to use it.
Update deprecation message to be consistent
Use `rspec-activemodel-mocks` if required.

Don't include RSpec::Rails::Mocks if RSpec::ActiveModel::Mocks exists.
This prevents the deprecation and runs the new code instead.
Try to require rspec-activemodel-mocks.