change to check README.rdoc instead

`rails new` command generates `README.rdoc` since Rails 4. So, I
would like to propose to change to checking the existence of
`README.rdoc` instead of creating `README` in
[Push Your App to GitHub](http://guides.railsgirls.com/github/).
Merge pull request #150 from eitoball/check_readme_rdoc_instead

Check README.rdoc instead