Django 1.8 warnings fix

Fixed warnings in development Django version:

```
/Users/smee/PycharmProjects/shantishop/system/venv/lib/python2.7/site-packages/easy_thumbnails/models.py:63: RemovedInDjango19Warning: Model class easy_thumbnails.models.Source doesn't declare an explicit app_label and either isn't in an application in INSTALLED_APPS or else was imported before its application was loaded. This will no longer be supported in Django 1.9.
  class Source(File):

WARNING  [2014-12-13 00:45:27,366] /Users/smee/PycharmProjects/shantishop/system/venv/lib/python2.7/site-packages/easy_thumbnails/models.py:63: RemovedInDjango19Warning: Model class easy_thumbnails.models.Source doesn't declare an explicit app_label and either isn't in an application in INSTALLED_APPS or else was imported before its application was loaded. This will no longer be supported in Django 1.9.
  class Source(File):

/Users/smee/PycharmProjects/shantishop/system/venv/lib/python2.7/site-packages/easy_thumbnails/models.py:67: RemovedInDjango19Warning: Model class easy_thumbnails.models.Thumbnail doesn't declare an explicit app_label and either isn't in an application in INSTALLED_APPS or else was imported before its application was loaded. This will no longer be supported in Django 1.9.
  class Thumbnail(File):

WARNING  [2014-12-13 00:45:27,368] /Users/smee/PycharmProjects/shantishop/system/venv/lib/python2.7/site-packages/easy_thumbnails/models.py:67: RemovedInDjango19Warning: Model class easy_thumbnails.models.Thumbnail doesn't declare an explicit app_label and either isn't in an application in INSTALLED_APPS or else was imported before its application was loaded. This will no longer be supported in Django 1.9.
  class Thumbnail(File):

/Users/smee/PycharmProjects/shantishop/system/venv/lib/python2.7/site-packages/easy_thumbnails/models.py:74: RemovedInDjango19Warning: Model class easy_thumbnails.models.ThumbnailDimensions doesn't declare an explicit app_label and either isn't in an application in INSTALLED_APPS or else was imported before its application was loaded. This will no longer be supported in Django 1.9.
  class ThumbnailDimensions(models.Model):

WARNING  [2014-12-13 00:45:27,369] /Users/smee/PycharmProjects/shantishop/system/venv/lib/python2.7/site-packages/easy_thumbnails/models.py:74: RemovedInDjango19Warning: Model class easy_thumbnails.models.ThumbnailDimensions doesn't declare an explicit app_label and either isn't in an application in INSTALLED_APPS or else was imported before its application was loaded. This will no longer be supported in Django 1.9.
  class ThumbnailDimensions(models.Model):
```