Add option to skip creation of test with TestUnit

For the purpose of this tutorial, because testing will be done with RSpec, wouldn't it be easier to append the `--skip-test-unit` option to avoid creating the `test` directory and all the associated files? And then we could remove line 87-97 of the article.
Update section under Using RSpec instead of TestUnit
Remove the last sentence from line 85

I missed that sentence in the previous pull request.
Add note for '--skip-test-unit' option to the rails command
Fix typo on line 55
Merge branch 'pr/605'
Merge branch 'pr/604'
Merge branch 'pr/603'