describeの書き方が変わったらしい。Myron Marston » RSpec 2.99 and 3.0 RC1 have been released!によると、「Issue a deprecation when described_class is accessed from within a nested describe <SomeClass> example group, since described_class will return the innermost described class in RSpec 3 rather than the outermost described class, as it behaved in RSpec 2. (Myron Marston)」とある。