Set model_dir to ["app/models"] if not set (fixes #265)

lib/tasks/annotate_models.rake

@@ -20,7 +20,7 @@ task :annotate_models => :environment do
   options[:show_foreign_keys] = Annotate.true?(ENV['show_foreign_keys'])
   options[:show_indexes] = Annotate.true?(ENV['show_indexes'])
   options[:simple_indexes] = Annotate.true?(ENV['simple_indexes'])
-  options[:model_dir] = ENV['model_dir'] ? ENV['model_dir'].split(',') : []
+  options[:model_dir] = ENV['model_dir'] ? ENV['model_dir'].split(',') : ['app/models']
   options[:include_version] = Annotate.true?(ENV['include_version'])
   options[:require] = ENV['require'] ? ENV['require'].split(',') : []
   options[:exclude_tests] = Annotate.true?(ENV['exclude_tests'])