install: Make sure python is installed before using it.

This is relevant for completely bare Ubuntu systems which might only
have python3 installed.

Fixes #40.

scripts/lib/install

@@ -12,7 +12,7 @@ EOF
 
 apt-get update
 apt-get -y dist-upgrade
-apt-get install -y puppet git
+apt-get install -y puppet git python
 
 mkdir -p /etc/zulip
 echo -e "[machine]\npuppet_classes = zulip::voyager\ndeploy_type = voyager" > /etc/zulip/zulip.conf