replace RDoc formatter with YARD

Rakefile

@@ -1,17 +1,16 @@
 require 'bundler/gem_tasks'
 require 'rspec/core/rake_task'
 require 'rspec/core'
-require 'rdoc/task'
+require 'yard'
 require 'rubocop/rake_task'
 
 RSpec::Core::RakeTask.new(:spec) do |spec|
   spec.pattern = FileList['spec/**/*_spec.rb'].uniq
   spec.rspec_opts = '--format documentation'
 end
 
-RDoc::Task.new(:doc) do |rdoc|
-  rdoc.main = 'README.rdoc'
-  rdoc.rdoc_files.include %w(README.rdoc lib)
+YARD::Rake::YardocTask.new(:doc) do |t|
+  t.files   = %w(README.rdoc lib)   # optional
 end
 
 RuboCop::RakeTask.new(:cop)

gnuplotrb.gemspec

@@ -22,7 +22,7 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency 'bundler', '~> 1.7'
   spec.add_development_dependency 'rake', '~> 10.0'
   spec.add_development_dependency 'rspec', '~> 3.2'
-  spec.add_development_dependency 'rdoc', '~> 4.2'
+  spec.add_development_dependency 'yard', '~> 0.8'
   spec.add_development_dependency 'rubocop', '~> 0.29'
   spec.add_development_dependency 'codeclimate-test-reporter'
   spec.add_development_dependency 'chunky_png'