[misc] fix deprecated rdoc rake tasks

Rakefile

@@ -96,8 +96,8 @@ task :coverage do
   sh "open coverage/index.html"
 end
 
-require 'rake/rdoctask'
-Rake::RDocTask.new do |rdoc|
+require 'rdoc/task'
+RDoc::Task.new do |rdoc|
   rdoc.rdoc_dir = 'rdoc'
   rdoc.title = "#{name} #{version}"
   rdoc.rdoc_files.include('README*')

fog.gemspec

@@ -49,6 +49,7 @@ Gem::Specification.new do |s|
   ## those that are only needed during development
   s.add_development_dependency('jekyll')
   s.add_development_dependency('rake')
+  s.add_development_dependency('rdoc')
   s.add_development_dependency('rspec', '1.3.1')
   s.add_development_dependency('shindo', '0.3.4')
   s.add_development_dependency('virtualbox', '0.8.3')