Only configure RSpec::Mocks if it is fully loaded

It's possible for the `::RSpec::Mocks` constant to exist, but for it not
to have the `configuration` method present. This can happen if another
library has required some files from _within_ the `rspec/mocks` library,
but nothing else has fully loaded it.

For a concrete example, see krisleech/wisper-rspec#21

While allowing external libraries to use parts of this one might not be
something that's explicitly supported, we can avoid some unexpected
exceptions by adding a little extra check to our detection of whether or
not the mocking library is loaded.

Author: lazyatom

lib/rspec/rails/active_record.rb

@@ -7,7 +7,7 @@ class ActiveRecordConfiguration
       def self.initialize_activerecord_configuration(config)
         config.before :suite do
           # This allows dynamic columns etc to be used on ActiveRecord models when creating instance_doubles
-          if defined?(ActiveRecord) && defined?(ActiveRecord::Base) && defined?(::RSpec::Mocks)
+          if defined?(ActiveRecord) && defined?(ActiveRecord::Base) && defined?(::RSpec::Mocks) && (::RSpec::Mocks.respond_to?(:configuration))
             ::RSpec::Mocks.configuration.when_declaring_verifying_double do |possible_model|
               target = possible_model.target