Set InstanceMethods#to_h as an alias.

ActiveRecord serializers require the to_h method. Without this PR, one must create a serializer like the following:

```ruby
class UserPreferences
  include Virtus.model

  attribute :github, String
  attribute :twitter, String
  attribute :newsletter, Boolean

  alias_method :to_h, :to_hash

  def self.dump(preferences)
    preferences.to_h
  end

  def self.load(preferences)
    new(preferences)
  end
end
```

The to_h method used to work on the `include Virtus` era, but not anymore.