Django 1.7 deprecations

[murphyke]

Although pretty minor, the Django 1.7 deprecations should be addressed. E.g.:

avocado/core/cache/managers.py:5: RemovedInDjango18Warning: `CacheManager.get_query_set` method should be renamed `get_queryset`.
  class CacheManager(models.Manager):

avocado/core/managers.py:6: RemovedInDjango18Warning: `PassThroughManager.get_query_set` method should be renamed `get_queryset`.
  class PassThroughManager(models.Manager):

avocado/core/managers.py:52: RemovedInDjango18Warning: `PublishedManager.get_query_set` method should be renamed `get_queryset`.
  class PublishedManager(PassThroughManager):

avocado/managers.py:212: RemovedInDjango18Warning: `DataFieldManager.get_query_set` method should be renamed `get_queryset`.
  class DataFieldManager(PublishedManager, DataFieldSearchMixin):

avocado/managers.py:253: RemovedInDjango18Warning: commit_on_success is deprecated in favor of atomic.
  def create_from_field(self, field, save=True, **kwargs):

avocado/managers.py:247: RemovedInDjango18Warning: `DataConceptManager.get_query_set` method should be renamed `get_queryset`.
  class DataConceptManager(PublishedManager, DataConceptSearchMixin):

avocado/history/managers.py:74: RemovedInDjango18Warning: commit_on_success is deprecated in favor of atomic.
  deleted=False, **kwargs):

avocado/history/managers.py:122: RemovedInDjango18Warning: commit_on_success is deprecated in favor of atomic.
  def revert(self, obj, model=None, commit=True):

avocado/history/managers.py:130: RemovedInDjango18Warning: commit_on_success is deprecated in favor of atomic.
  def cull_for_object(self, obj, model=None, max_size=None):

avocado/admin.py:55: RemovedInDjango18Warning: Creating a ModelForm without either the 'fields' attribute or the 'exclude' attribute is deprecated - form DataFieldAdminForm needs updating
  class DataFieldAdminForm(forms.ModelForm):

avocado/admin.py:175: RemovedInDjango18Warning: commit_on_success is deprecated in favor of atomic.
  @transaction.commit_on_success