Change additional path from Pathname to String

Because some gems expects that autoload_paths is Array of String

For example `sunspot-rails`
```
% rake db:migrate
rake aborted!
undefined method `=~' for #<Pathname:/home/joker/rails_app/app/views>
/home/joker/rails_app/vendor/bundle/ruby/2.0.0/gems/sunspot_rails-2.0.0/lib/sunspot/rails/tasks.rb:70:in `block in sunspot_solr_in_load_path?'
/home/joker/rails_app/vendor/bundle/ruby/2.0.0/gems/sunspot_rails-2.0.0/lib/sunspot/rails/tasks.rb:70:in `each'
/home/joker/rails_app/vendor/bundle/ruby/2.0.0/gems/sunspot_rails-2.0.0/lib/sunspot/rails/tasks.rb:70:in `any?'
/home/joker/rails_app/vendor/bundle/ruby/2.0.0/gems/sunspot_rails-2.0.0/lib/sunspot/rails/tasks.rb:70:in `sunspot_solr_in_load_path?'
/home/joker/rails_app/vendor/bundle/ruby/2.0.0/gems/sunspot_rails-2.0.0/lib/sunspot/rails/tasks.rb:73:in `block in <top (required)>'
/home/joker/rails_app/vendor/bundle/ruby/2.0.0/gems/rake-10.0.4/lib/rake/task_manager.rb:196:in `in_namespace'
/home/joker/rails_app/vendor/bundle/ruby/2.0.0/gems/rake-10.0.4/lib/rake/dsl_definition.rb:104:in `namespace'
/home/joker/rails_app/vendor/bundle/ruby/2.0.0/gems/sunspot_rails-2.0.0/lib/sunspot/rails/tasks.rb:1:in `<top (required)>'
/home/joker/rails_app/vendor/bundle/ruby/2.0.0/gems/activesupport-3.2.12/lib/active_support/dependencies.rb:251:in `require'
/home/joker/rails_app/vendor/bundle/ruby/2.0.0/gems/activesupport-3.2.12/lib/active_support/dependencies.rb:251:in `block in require'
/home/joker/rails_app/vendor/bundle/ruby/2.0.0/gems/activesupport-3.2.12/lib/active_support/dependencies.rb:236:in `load_dependency'
/home/joker/rails_app/vendor/bundle/ruby/2.0.0/gems/activesupport-3.2.12/lib/active_support/dependencies.rb:251:in `require'
/home/joker/rails_app/Rakefile:12:in `<top (required)>'

```