v9.15.0

• Fix bug where history would not completely flush all transactions,
  causing store handlers to fire twice.
• start() must be invoked

Potentially breaking changes

In a previous update, we made a change that allowed instances of microcosm to work without invoking start(). This update reverts that decision. Without intentionaly invoking start, transactional state becomes hard to predict. This is potentially a breaking change; for those upgrading, verify that you are calling start before using a microcosm.