Add --skip-missing-interpreters to drone

.drone.yml

@@ -4,6 +4,6 @@ env:
   - MYSQL_URI=mysql://root@127.0.0.1:3306/
 script:
   - pip install tox
-  - tox -e "{py27,py33,py34,py35}-{sqlite,postgresql}"
+  - tox -e "{py27,py33,py34,py35}-{sqlite,postgresql}" --skip-missing-interpreters
 services:
   - postgres