Only do package update, no system upgrade

The guides already say to to an upgrade before even running this script and the way this works right now it just hangs for a long time without any feedback for sometimes 10-20 minutes.
This is I think not a very good UX. See this issue on more reasoning: adafruit#174
The default nature of upgrading all the system packages is I think a bit too many changes than one would expect from installing a python package.

raspi-blinka.py

@@ -38,9 +38,6 @@ def sys_update():
     print("Updating System Packages")
     if not shell.run_command("sudo apt-get update"):
         shell.bail("Apt failed to update indexes!")
-    print("Upgrading packages...")
-    if not shell.run_command("sudo apt-get -y upgrade"):
-        shell.bail("Apt failed to install software!")
 
 def set_raspiconfig():
     """