Typo in comment removed
Cleanup compares against the last pulled image ID

- This should be better at cleaning out stopped containers from old images
Container destroys now only happen in cleanup
Attempting to remove the old image after a pull was silly

Immediately after pulling, the likelihood that there are containers using the old image
is around 100%, considering that's the whole point of this app.
WEBDRIVER_WHARF_POOL_SIZE must be an int