Use round

Without this change, it seems that VariableExchange#exchange_to is not as accurate it could be.

The Float#to_s.to_i will turn `1.9` into `1`

This change replaces `Float#to_s.to_i` to `Float#round`. 

I'm not sure why we are using float in the first place though on the first line, but I have not changed it as it will change the external API.