fix 'uninitialized instance variable' warning

Running specs in my Rails 4.1.2.rc1 app with warnings turned on prints this warning, and just this warning. So I fixed it. :)

Author: indirect

lib/rspec/rails/adapters.rb

@@ -120,6 +120,10 @@ def teardown(*methods)
         end
       end
 
+      def initialize
+        @example = nil
+      end
+
       def method_name
         @example
       end