Double alias problem

Hi...

I've found a bug (#70)... it seems that rubocop (version 0.5.0) doesn't like so much a snippet of code like this one:

```ruby    
alias has_foo? foo?
alias has_bar? bar?
```

I've make a spec (maybe could be useful) and a fix (that is probably too much _naive_) but I hope this could be useful in some way...