BUG: Apply integer truncation to order amounts earlier in the pipeline.

Truncate non-integer order amounts in `TradingAlgorithm.order` instead of
`Blotter.order`.  This fixes an issue where non-integer orders coming out of
order_value can spuriously trigger a `LongOnly` trading guard.

Example:

sid.price == 2.0
order_value(sid, 5) -> order(sid, 2.5) -> truncated to order(sid, 2.0)
order_value(sid, -5) -> order(sid, -2.5) -> LongOnlyViolation b/c 2.0 - 2.5 < 0