force django 1.9.0
force django 1.9.0

signalserver breaks with version 1.11, see
https://github.com/bavc/signalserver/issues/226.

fixes https://github.com/bavc/signalserver/issues/226

yayoiukai/signalserver,yayoiukai/signalserver,yayoiukai/signalserver,yayoiukai/signalserver