Emit deprecation warning for running_example

Note that we set this up only for the "running_example" getter, not the
setter, as the latter should only ever be used internally by RSpec
itself, not by external callers.

lib/rspec/core/example_group.rb

@@ -8,6 +8,10 @@ class ExampleGroup
 
       attr_accessor :example
 
+      def running_example
+        RSpec.deprecate('running_example', 'example')
+      end
+
       def self.world
         RSpec.world
       end

spec/rspec/core/deprecations_spec.rb

@@ -7,4 +7,13 @@
       Spec
     end
   end
+
+  describe RSpec::Core::ExampleGroup do
+    describe 'running_example' do
+      it 'is deprecated' do
+        RSpec.should_receive(:warn_deprecation).with /running_example.*example/m
+        self.running_example
+      end
+    end
+  end
 end