Fix undefined local variable amount_of_retries for ViddlRb::DownloadHelper

I faced 403 errors from Youtube and the retry loop failed because of a coding oops :

```
Error: undefined local variable or method `amount_of_retries' for ViddlRb::DownloadHelper:Class

Backtrace:
/home/maxime/.rvm/gems/ruby-1.9.3-p327@[...]/gems/viddl-rb-0.75/helper/download-helper.rb:45:in `block in save_file'
[...]
```