Updating marshal to exclude hydra

Mashalling a ` Typhoeus::Request ` instance fails if the request was performed through hydra. Hydra's @multi could not be marshalled so I just removed @hydra altogether from the request when marshalling.

To reproduce the error:
```ruby
> hydra = Typhoeus::Hydra.new
> request = Typhoeus::Request.new('http://www.google.com')
> hydra.queue(request)
> hydra.run
> Marshal.dump(request)
=> TypeError: no marshal_dump is defined for class FFI::MemoryPointer
```