BLD: Upgrade numpy from 1.8.0 -> 1.8.1

Keep numpy package up to date.
1.8.1 is a bug fix release, so should be no breaking changes.
MAINT: Make the portfolio property call `updated_portfolio`.

Make the portfolio property on TradingAlgorithm call `updated_portfolio`
internally.  This prevents needless recomputation of the portfolio between
calls to `handle_data`, and also prevents issues where the portfolio object
could be unexpectedly modified in place in the body of a `handle_data` call.