tox: Use custom install command

7 jobs for master in 31 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #61565
py27-django110

00:00:15

89.0%
passed #61566
py27-django111

00:00:16

89.0%
passed #61564
py27-django18

00:00:15

89.0%
passed #61568
py36-django110

00:00:17

90.0%
passed #61569
py36-django111

00:00:13

90.0%
passed #61567
py36-django18

00:00:18

90.0%
passed #61570
py36-django20

00:00:13

90.0%