Test that devnull permissions don't kill us

A test for the issue described in matthewwithanm/django-imagekit#294
(dev/null permissions break the `quiet` decorator).
Fail gracefully if dev/null isn't writeable

Related: matthewwithanm/django-imagekit#294
Merge branch 'devnull-perms'

* devnull-perms:
  Fail gracefully if dev/null isn't writeable
  Test that devnull permissions don't kill us
Bump to version 1.1.12