Ensure logging format strings are unicode

Fixes #1035
Signed-off-by: Michal Čihař <michal@cihar.com>

weblate/trans/mixins.py

@@ -18,6 +18,8 @@
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 #
 
+from __future__ import unicode_literals
+
 import os
 
 from django.core.urlresolvers import reverse