No files checked when the path contains a directory excluded by default

If the specified path contains one of the directories that can be excluded (vendor, spec, test, features, and tmp), then the "parse_files" method filters out all of the files, and then reports that there are no errors because no files were checked.

```
[localhost scadd]$ pwd
/Users/me/tmp/scadd
[localhost scadd]$ rails_best_practices --silent .
./app/controllers/application_controller.rb:3 - remove trailing whitespace

Please go to http://rails-bestpractices.com to see more useful Rails Best Practices.

Found 1 warnings.
[locahost scadd]$ rails_best_practices --silent /Users/me/tmp/scadd

Please go to http://rails-bestpractices.com to see more useful Rails Best Practices.

No warning found. Cool!
```

I'll fix this and submit a pull request in the next day or two.