Add missing stdlib Set require to ClassAttribute.

Ran into this trying to `require 'lotus/utils/class_attributes'`:

    NameError: uninitialized constant Lotus::Utils::ClassAttribute::ClassMethods::Set
    from /usr/local/var/rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/lotus-utils-0.1.0/lib/lotus/utils/class_attribute.rb:89:in `class_attributes'