Version key does not work on Google Play

Using version codes (integers) on version field does not work, client still tries to install latest version. "vc" key on "/fdfe/purchase" endpoint should be overriden with the given key. I know that the version number needs to be known to use this but this could be really useful for diffing between versions.

Update: It seems like gpapi supports this already, as seen [here](https://github.com/EFForg/rs-google-play/blob/4cd37b517d145ac6a7148a2a14c72c7151a3fcea/gpapi/src/lib.rs#L242)