undefined method `underscore' for ActiveModel

Since the ActiveModel::Name in Rails 4 is not a String anymore before to get the model name to generate the sitemap, it should be changed to string

```ruby
name = options.delete(:name) || collection.model_name.underscore.pluralize.to_sym
```
because in Rails 4 it throw the next exception:

```
Generating sitemap...
I, [2013-07-31T17:43:59.899011 #6631]  INFO -- : Generating sitemap...
rake aborted!
undefined method `underscore' for #<ActiveModel::Name:0x007f95cc8e7290>
/Users/joselo/.rvm/gems/ruby-2.0.0-p195@bolido/gems/dynamic_sitemaps-2.0.0.beta2/lib/dynamic_sitemaps/generator.rb:61:in `sitemap_for'
```