Remove use of deprecated RubyGems #has_rdoc method

As of RubyGems 1.7.0, the #has_rdoc= method is deprecated.  

1.7.0 Release notes: http://blog.segment7.net/2011/04/01/rubygems-1-7-0

Prior to this commit, requiring the simple-rss gem or running rake for the simple-rss gem would produce the following warning:

    NOTE: Gem::Specification#has_rdoc= is deprecated with no replacement. It
    will be removed on or after 2011-10-01.