gitian: Need ca-certificates and python for LXC builds

doc/gitian-building.md

@@ -259,7 +259,7 @@ adduser debian sudo
 Then set up LXC and the rest with the following, which is a complex jumble of settings and workarounds:
 
 ```bash
-# the version of lxc-start in Debian 7.4 needs to run as root, so make sure
+# the version of lxc-start in Debian needs to run as root, so make sure
 # that the build script can execute it without providing a password
 echo "%sudo ALL=NOPASSWD: /usr/bin/lxc-start" > /etc/sudoers.d/gitian-lxc
 # make /etc/rc.local script that sets up bridge between guest and host