Remove call to rspec-core internals from rspec/doubles.rb

RSpec::Core::MockFrameworkAdapter is not intended to be used outside
rspec-core itself, RSpec::Mocks::ExampleMethods should fulfil the
same purpose.

See rspec/rspec-core#1480
Merge pull request #666 from lime/rspec_mock_adapter_call

Remove call to rspec-core internals from rspec/doubles.rb