Remove volumes from drone and fix test

.drone.yml

@@ -13,8 +13,6 @@ pipeline:
         commands:
             - pip install tox
             - tox -e "${TOXENV}-${DATABASE}"
-        volumes:
-            - cache:/root/.cache
 
 services:
     postgresql:

tox.ini

@@ -15,4 +15,4 @@ setenv =
     sqlite: DB_NAME={env:SQLITE_NAME::memory:}
     postgresql: DB_NAME={env:POSTGRESQL_NAME:test}
     mysql: DB_NAME={env:MYSQL_NAME:test}
-install_command = pip install --pre --find-links https://trydevpi.tryton.org/ {opts} {packages}
+install_command = pip install --pre --find-links https://trydevpi.tryton.org/ --process-dependency-links {opts} {packages}