Require all of rss when importing RSS feed.

Otherwise it won't parse an Atom field.

Fixes #194.

Author: parkr

lib/jekyll-import/importers/rss.rb

@@ -13,6 +13,7 @@ def self.validate(options)
 
       def self.require_deps
         JekyllImport.require_with_fallback(%w[
+          rss
           rss/1.0
           rss/2.0
           open-uri