ec2/bootstrap.sh: support xenial if debootstrap version is too old
ec2/bootstrap.sh: support xenial if debootstrap version is too old

The deboostrap version of wheezy-backports doesn't support
Ubuntu/xenial yet, since all the recent Ubuntu versions
are supported by the gutsy script itself just make it a symlink
as usual.

Closes #6

sipwise/kamailio-deb-jenkins,sipwise/kamailio-deb-jenkins