Spec behaviour of Ruby 2.0's Module prepension

As of Ruby 2.0.0p0 you can call Class#prepend to insert a module at the
beginning of the ancestor chain (effectively allowing modules to
override instance methods).

For example:

```ruby
klass = Class.new do
  def test
   "foo"
  end
end

mod = Module.new do
  def test
    "hello #{super}"
  end
end

klass.send(:prepend, mod)

klass.ancestors
=> [#<Module:0x007f922e59a4a8>, #<Class:0x007f922e5718f0>, Object, Kernel, BasicObject]

klass.new.test
=> "hello foo"
```

This spec verifies behaviour of calling prepend on a Class and that the
Module#prepended hook callback is called.