Install sqlitebck on drone

issue5794

.drone.yml

@@ -3,7 +3,7 @@ env:
   - POSTGRESQL_URI=postgresql://postgres@127.0.0.1:5432/
   - MYSQL_URI=mysql://root@127.0.0.1:3306/
 script:
-  - pip install tox
+  - pip install tox sqlitebck
   - tox -e "{py27,py33,py34,py35}-{sqlite,postgresql}" --skip-missing-interpreters
 services:
   - postgres