Fixed `const_missing': uninitialized constant RDoc::Store (NameError)

Added rspec-kickstarter to Rails 4.0.0 projects Gemfile.
Execute the following command, and get the const_missing error.

```
$ bundle exec rspec-kickstarter -r app
/xxx/vendor/bundle/ruby/2.0.0/gems/rdoc-3.12.2/lib/rdoc.rb:105:in `const_missing': uninitialized constant RDoc::Store (NameError)
	from /xxx/vendor/bundle/ruby/2.0.0/gems/rspec-kickstarter-0.2.4/lib/rspec_kickstarter/generator.rb:118:in `get_ruby_parser'
```

My environments are 
- ruby 2.0.0
- RDoc 3.12.2
  - sdoc depends on RDoc version 3.x