`rubocop` against RuboCop project

With `<< line_numer`:

```
Finished in 12.515619 seconds
Finished in 12.589461 seconds
Finished in 12.423268 seconds
Finished in 12.604064 seconds
Finished in 12.581377 seconds
Finished in 12.506494 seconds
Finished in 12.441493 seconds
Finished in 12.41333 seconds
Finished in 12.449793 seconds
Finished in 12.65696 seconds
```

With `+= [line_number]`:

```
Finished in 12.714099 seconds
Finished in 12.84459 seconds
Finished in 12.794755 seconds
Finished in 12.683052 seconds
Finished in 12.666977 seconds
Finished in 12.605673 seconds
Finished in 12.840705 seconds
Finished in 12.783094 seconds
Finished in 12.727988 seconds
Finished in 12.74918 seconds
```