Builds are not always in the right order

[ryandesign (Ryan Carsten Schmidt)]

Curiously builds aren't always in the right order.

For example ​this portwatcher build of py-boto3 had a port list of py27-boto3 py-boto3 py35-boto3 py36-boto3 py37-boto3. That's from mpbb list-subports and is in the correct order.

But the ​portbuilder build of py-boto3 ran first, even though it was assigned the correct priority of 2, meaning it should have gone second. The ​build of py27-boto3 with priority 1 that ran afterward should have gone first.