MAINT: Use current_data instead of last_sale_price

It seems more clear to get price values from
`self.trading_client.current_data[sid].price` than
from `self.portfolio.positions[sid].last_sale_price`.

The two values are the same, so this is just a readability change,
but it is also the same behavior as in `self.order_value()` and it's
good to have them all be the same.