improve ruby-2.5 compatibility (epel8)
    
    An unfortunate bug in rdoc-5.0 causes a build failure on epel8.  This is
    fixed in ruby-2.6/rdoc-6.0.2¹.  Until/unless the fix is backported to
    RHEL-8 (which seems unlikely), we can only patch around this.
    
    Thanks, as usual, to Dan Allen for going above and beyond to patch this
    upstream.
    
    ¹ https://github.com/ruby/rdoc/pull/586
      https://github.com/ruby/rdoc/commit/a1631aa