Install prettytable>=0.7 to satisfy pip 6/PEP 440

Also use sudo -H with pip so that it doesn't create a ~stack/.cache
other things can't write to as the stack user later.

Change-Id: I2134c7d8f58f8b83f33150c9ed86d87f8ccba2f3

functions-common

@@ -1578,7 +1578,7 @@ function pip_install {
         local sudo_pip="env"
     else
         local cmd_pip=$(get_pip_command)
-        local sudo_pip="sudo"
+        local sudo_pip="sudo -H"
     fi
 
     $xtrace

tools/fixup_stuff.sh

@@ -85,7 +85,7 @@ function get_package_path {
 
 # Fix prettytable 0.7.2 permissions
 # Don't specify --upgrade so we use the existing package if present
-pip_install 'prettytable>0.7'
+pip_install 'prettytable>=0.7'
 PACKAGE_DIR=$(get_package_path prettytable)
 # Only fix version 0.7.2
 dir=$(echo $PACKAGE_DIR/prettytable-0.7.2*)