Update the old filebasedstorage imports in migrations to use Django's default file storage.

Signed-off-by: Roberto Rosario <roberto.rosario.gonzalez@gmail.com>

mayan/apps/document_signatures/migrations/0001_initial.py

@@ -2,7 +2,8 @@
 from __future__ import unicode_literals
 
 from django.db import models, migrations
-import storage.backends.filebasedstorage
+from django.core.files.storage import FileSystemStorage
+
 import document_signatures.models
 
 
@@ -24,7 +25,7 @@ class Migration(migrations.Migration):
                 ),
                 (
                     'signature_file', models.FileField(
-                        storage=storage.backends.filebasedstorage.FileBasedStorage(),
+                        storage=FileBasedStorage(),
                         upload_to=document_signatures.models.upload_to,
                         blank=True, editable=False, null=True,
                         verbose_name='Signature file'