[FIX] point_of_sale: hardcode jessie in posbox scripts

Before this commit, the latest version of Raspbian was chosen.
It is not very helpful though as if something goes wrong, we don't on which part it is.

Also, fix python dependencies for the image

addons/point_of_sale/tools/posbox/overwrite_before_init/etc/init_posbox_image.sh

@@ -17,8 +17,7 @@ export DEBIAN_FRONTEND=noninteractive
 echo "nameserver 8.8.8.8" >> /etc/resolv.conf
 
 
-apt-get update
-apt-get -y dist-upgrade
+apt-get update && apt-get -y upgrade
 # Do not be too fast to upgrade to more recent firmware and kernel than 4.38
 # Firmware 4.44 seems to prevent the LED mechanism from working
 
@@ -39,6 +38,8 @@ rm -rf /usr/share/doc
 pip install pyusb==1.0.0b1
 pip install qrcode
 pip install evdev
+pip install simplejson
+pip install unittest2
 
 # --upgrade because websocket_client in wheezy is bad:
 # https://github.com/docker/compose/issues/1288

addons/point_of_sale/tools/posbox/posbox_download_images.sh

@@ -1,5 +1,5 @@
 #!/bin/sh
 
-wget 'https://downloads.raspberrypi.org/raspbian_lite_latest' -O raspbian.img.zip
+wget 'http://downloads.raspberrypi.org/raspbian_lite/images/raspbian_lite-2017-07-05/2017-07-05-raspbian-jessie-lite.zip' -O raspbian.img.zip
 unzip raspbian.img.zip
 wget 'https://github.com/dhruvvyas90/qemu-rpi-kernel/raw/master/kernel-qemu-4.4.13-jessie' -O kernel-qemu