Updating marshal to exclude on_success and on_failure

Mashalling a ` Typhoeus::Request ` instance fails if there is a ` on_success ` or a ` on_failure ` callback defined. I updated the ` Marshal ` module to also exclude these handlers.

To reproduce the error:
```ruby
> request = Typhoeus::Request.new('http://www.google.com')
> request.on_success { }
> Marshal.dump(request)
=> TypeError: no marshal_dump is defined for class Proc
```