BUG: Ensure consistent ordering of amounts, prices & multipliers

Previously, we have assumed that the `amounts` and `last_sale_prices`
lists have the same order as the `value_multipliers`. This is not
correct, since to populate the `amounts` and `last_sale_prices` lists
we iterate over a `dict` (self.positions). The order of this `dict`
can change in arbitrary ways when it is updated, which occurs when
we call `update_positions`. Our `value_multipliers` however are stored
in an `OrderedDict`, meaning the order of existing key/value pairs
is not changed when they are updated.

To address this issue, we make sure that `self.positions` subclasses
`OrderedDict`.

zipline/finance/performance/position.py

@@ -33,6 +33,7 @@ Position Tracking
 
 from __future__ import division
 from math import copysign
+from collections import OrderedDict
 
 from copy import copy
 
@@ -228,7 +229,7 @@ last_sale_price: {last_sale_price}"
         self.__dict__.update(state)
 
 
-class positiondict(dict):
+class positiondict(OrderedDict):
 
     def __missing__(self, key):
         pos = Position(key)