Ensure we have locales

It seems Django needs them:
https://code.djangoproject.com/ticket/21991Signed-off-by: Michal Čihař <michal@cihar.com>

.travis.yml

@@ -54,3 +54,8 @@ sudo: false
 notifications:
   webhooks:
     - https://buildtimetrend.herokuapp.com/travis
+# install build dependencies
+addons:
+  apt:
+    packages:
+    - locales