Correcting the pattern style in example on README

I noticed that the example in the readme uses a pattern style that is deprecated. This corrects that. 

I am also using strings on all the filepaths to keep things consistent. Without that `**/Rakefile` will cause a parsing error. 

```
rubocop -V
0.23.0 (using Parser 2.1.9, running on ruby 2.1.2 x86_64-darwin13.0)
```

If this is something that you want a change log entry for, I can add it in.