Use timezone dates

Fixes #114

fedireads/routine_book_tasks.py

@@ -1,11 +1,12 @@
 ''' Routine tasks for keeping your library tidy '''
-from datetime import datetime, timedelta
+from datetime import timedelta
+from django.utils import timezone
 from fedireads import books_manager
 from fedireads import models
 
 def sync_book_data():
     ''' update books with any changes to their canonical source '''
-    expiry = datetime.now() - timedelta(days=1)
+    expiry = timezone.now() - timedelta(days=1)
     books = models.Book.objects.filter(
         sync=True,
         last_sync_date__lte=expiry