Remove deprecated gemspec properties

Avoids the following warnings when building the gem: ``` NOTE: Gem::Specification#rubyforge_project= is deprecated with no replacement. It will be removed on or after 2019-12-01. Gem::Specification#rubyforge_project= called from ruby-graphviz.gemspec:23. NOTE: Gem::Specification#has_rdoc= is deprecated with no replacement. It will be removed on or after 2018-12-01. Gem::Specification#has_rdoc= called from ruby-graphviz.gemspec:24. ```

Author: deivid-rodriguez

ruby-graphviz.gemspec

@@ -20,8 +20,6 @@ Gem::Specification.new do |s|
  s.executables = `git ls-files -- bin/*`.split("\n").map{ |f| File.basename(f) }
  s.require_paths = ["lib"]
 
- s.rubyforge_project = 'ruby-asp'
- s.has_rdoc = true
  s.extra_rdoc_files = ["README.rdoc", "COPYING.rdoc", "AUTHORS.rdoc", "CHANGELOG.rdoc"]
  s.rdoc_options = ["--title", "Ruby/GraphViz", "--main", "README.rdoc"]
  s.post_install_message = %{