title: django-simple-history 1.9 break unittests
body: <p>When I run unittests with django-simple-history==1.6.0, it runs fine but with
  django-simple-history==1.9.0 it breaks the unittests.
  Any ideas how to fix this?</p>
  
  <pre><code>Creating test database for alias 'default'...
  Traceback (most recent call last):
    File "manage.py", line 12, in &lt;module&gt;
      execute_from_command_line(sys.argv)
    File "/home/me/venv/lib/python2.7/site-packages/django/core/management/__init__.py", line 338, in execute_from_command_line
      utility.execute()
    File "/home/me/venv/lib/python2.7/site-packages/django/core/management/__init__.py", line 330, in execute
      self.fetch_command(subcommand).run_from_argv(self.argv)
    File "/home/me/venv/lib/python2.7/site-packages/django/core/management/commands/test.py", line 30, in run_from_argv
      super(Command, self).run_from_argv(argv)
    File "/home/me/venv/lib/python2.7/site-packages/django/core/management/base.py", line 390, in run_from_argv
      self.execute(*args, **cmd_options)
    File "/home/me/venv/lib/python2.7/site-packages/django/core/management/commands/test.py", line 74, in execute
      super(Command, self).execute(*args, **options)
    File "/home/me/venv/lib/python2.7/site-packages/django/core/management/base.py", line 441, in execute
      output = self.handle(*args, **options)
    File "/home/me/venv/lib/python2.7/site-packages/django/core/management/commands/test.py", line 90, in handle
      failures = test_runner.run_tests(test_labels)
    File "/home/me/venv/lib/python2.7/site-packages/django/test/runner.py", line 210, in run_tests
      old_config = self.setup_databases()
    File "/home/me/venv/lib/python2.7/site-packages/django/test/runner.py", line 166, in setup_databases
      **kwargs
    File "/home/me/venv/lib/python2.7/site-packages/django/test/runner.py", line 370, in setup_databases
      serialize=connection.settings_dict.get("TEST", {}).get("SERIALIZE", True),
    File "/home/me/venv/lib/python2.7/site-packages/django/db/backends/base/creation.py", line 376, in create_test_db
      self.connection._test_serialized_contents = self.serialize_db_to_string()
    File "/home/me/venv/lib/python2.7/site-packages/django/db/backends/base/creation.py", line 413, in serialize_db_to_string
      serializers.serialize("json", get_objects(), indent=None, stream=out)
    File "/home/me/venv/lib/python2.7/site-packages/django/core/serializers/__init__.py", line 129, in serialize
      s.serialize(queryset, **options)
    File "/home/me/venv/lib/python2.7/site-packages/django/core/serializers/base.py", line 52, in serialize
      for obj in queryset:
    File "/home/me/venv/lib/python2.7/site-packages/django/db/backends/base/creation.py", line 409, in get_objects
      for obj in queryset.iterator():
    File "/home/me/venv/lib/python2.7/site-packages/django/db/models/query.py", line 238, in iterator
      results = compiler.execute_sql()
    File "/home/me/venv/lib/python2.7/site-packages/django/db/models/sql/compiler.py", line 829, in execute_sql
      cursor.execute(sql, params)
    File "/home/me/venv/lib/python2.7/site-packages/django/db/backends/utils.py", line 64, in execute
      return self.cursor.execute(sql, params)
    File "/home/me/venv/lib/python2.7/site-packages/django/db/utils.py", line 97, in __exit__
      six.reraise(dj_exc_type, dj_exc_value, traceback)
    File "/home/me/venv/lib/python2.7/site-packages/django/db/backends/utils.py", line 64, in execute
      return self.cursor.execute(sql, params)
    File "/home/me/venv/lib/python2.7/site-packages/django/db/backends/sqlite3/base.py", line 318, in execute
      return Database.Cursor.execute(self, query, params)
  django.db.utils.OperationalError: no such column: myapp_historicalbook.history_change_reason
  </code></pre>