Ruby 2.1 Module#prepend isn't private

Ruby 2.0.0:

```
>> Module.private_method_defined?(:prepend)
=> true
>> Module.method_defined?(:prepend)
=> false
```

Ruby 2.1.1:

```
>> Module.private_method_defined?(:prepend)
=> false
>> Module.method_defined?(:prepend)
=> true
```

This causes [this check](https://github.com/rspec/rspec-support/blob/master/lib/rspec/support/ruby_features.rb#L24) to fail on Ruby 2.1.