Declare app_label in model Meta class to work with Django 1.9

Fixes RemovedInDjango19Warning:

Model class db_mutex.models.DBMutex 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.

db_mutex/models.py

@@ -15,4 +15,4 @@ class DBMutex(models.Model):
     creation_time = models.DateTimeField(auto_now_add=True)
 
     class Meta:
-        app_label = 'db_mutex'
+        app_label = "db_mutex"