Auto merge of #1695 - rubygems:update_rdoc, r=segiddins

Use rdoc ~> 4.0

# Description:
  closes #1692 seems is an issue with rdoc 3 updating it to rdoc 4 fixed it.
# Tasks:

- [ ] Describe the problem / feature
- [ ] Write tests
- [ ] Write code to solve the problem
- [ ] Get code review from coworkers / friends
- [ ] [Squash commits](http://gitready.com/advanced/2009/02/10/squashing-commits-with-rebase.html)

I will abide by the [code of conduct](https://github.com/rubygems/rubygems/blob/master/CODE_OF_CONDUCT.md).

Rakefile

@@ -58,7 +58,7 @@ hoe = Hoe.spec 'rubygems-update' do
 
   dependency 'builder',       '~> 2.1',   :dev
   dependency 'hoe-seattlerb', '~> 1.2',   :dev
-  dependency 'rdoc',          '~> 3.0',   :dev
+  dependency 'rdoc',          '~> 4.0',   :dev
   dependency 'ZenTest',       '~> 4.5',   :dev
   dependency 'rake',          '~> 10.5',  :dev
   dependency 'minitest',      '~> 4.0',   :dev
@@ -102,7 +102,7 @@ Rake::Task['docs'].clear
 Rake::Task['clobber_docs'].clear
 
 begin
-  gem 'rdoc', '~> 3.0'
+  gem 'rdoc', '~> 4.0'
   require 'rdoc/task'
 
   RDoc::Task.new :rdoc => 'docs', :clobber_rdoc => 'clobber_docs' do |doc|