Empty MLHS: SyntaxError

Hehe, as you suggested in #134, I refactored
`proc { |(a, _)| }`
to
`proc { |(a)| }`
but this time one of the mutations really isn't killable :P

```
/home/lars/.gem/ruby/1.9.3/bundler/gems/mutant-02ebe7261a49/lib/mutant/loader.rb:46:in `eval': /home/lars/workspace/hatetepe/lib/hatetepe/client.rb:50: syntax error, unexpected ')' (SyntaxError)
      @requests.delete_if do |()|
                                ^
/home/lars/workspace/hatetepe/lib/hatetepe/client.rb:56: syntax error, unexpected keyword_end, expecting $end
end # Hatetepe
   ^
```