update money dependency to 6.0.1

I thought this was worth it since the money 6.0.1 is out of beta. I wasn't entirely sure if using `>=` or `~>` was preferable. money-rails seems to use `~>` but it looks like this gem has historically favored `>=`.

Maybe a good time to release an updated gem, to get it on money 6.0 like the other gems?