Merge pull request #527 from foosel/hotfix/downgrade-pip

Pin pip to 9.0.3 until OctoPrint 1.3.9 is released

Author: guysoft

src/modules/octopi/start_chroot_script

@@ -36,7 +36,8 @@ pushd /home/pi
   sudo -u pi virtualenv oprint
 
   # first lets upgrade pip (the shipped version is way too old...)
-  sudo -u pi /home/pi/oprint/bin/pip install --upgrade pip
+  # issue #526 - pin pip to 9.0.3 until OctoPrint 1.3.9 is released
+  sudo -u pi /home/pi/oprint/bin/pip install pip==9.0.3
 
   # OctoPrint & pyserial
   if [ "$OCTOPI_INCLUDE_OCTOPRINT" == "yes" ]