Remove obsolete ZODB snapshot tarball
Update to latest bootstrap.py

Downloaded from https://bootstrap.pypa.io/bootstrap-buildout.py
Allow test layer teardown

For some reason zope.testrunner is unable to use subprocesses under tox,
so not allowing teardown breaks tests in tox.

(Teardown is not enabled by default because of BBB and fear that some
apps might do Things in layer setup that won't be torn down by the
default CleanUp.tearDown().)