Fix for ckan 2.9 and Revision exception

[davidread]

Test errors on master appear to be unrelated to this PR. The revision one is clearly to do with master/2.9 not having revisions any more.

======================================================================

ERROR: test_archiver.TestDownload.test_download_file

----------------------------------------------------------------------

Traceback (most recent call last):

  File "/home/travis/virtualenv/python2.7.13/lib/python2.7/site-packages/nose/case.py", line 384, in tearDown

    try_run(self.inst, ('teardown', 'tearDown'))

  File "/home/travis/virtualenv/python2.7.13/lib/python2.7/site-packages/nose/util.py", line 471, in try_run

    return func()

  File "/home/travis/build/ckan/ckanext-archiver/tests/test_archiver.py", line 405, in teardown

    model.repo.new_revision()

AttributeError: Repository instance has no attribute 'new_revision'

======================================================================

ERROR: test_archiver.TestDownload.test_head_unsupported

----------------------------------------------------------------------

Traceback (most recent call last):

  File "/home/travis/virtualenv/python2.7.13/lib/python2.7/site-packages/nose/case.py", line 197, in runTest

    self.test(*self.arg)

  File "/home/travis/build/ckan/ckanext-archiver/tests/test_archiver.py", line 63, in decorated

    return func(*(args + ('%s/%s' % (serveraddr, url),)), **kwargs)

  File "/home/travis/build/ckan/ckanext-archiver/tests/test_archiver.py", line 421, in test_head_unsupported

    resource = self._test_resource(url)

  File "/home/travis/build/ckan/ckanext-archiver/tests/test_archiver.py", line 414, in _test_resource

    pkg = get_action('package_create')(context, pkg)

  File "/home/travis/build/ckan/ckanext-archiver/ckan/ckan/logic/__init__.py", line 464, in wrapped

    result = _action(context, data_dict, **kw)

  File "/home/travis/build/ckan/ckanext-archiver/ckan/ckan/logic/action/create.py", line 178, in package_create

    raise ValidationError(errors)

ValidationError: {'name': ['That URL is already in use.']}