Add travis user to www-data user so travis user can chgrp files to www-data

[jonpugh]

Aegir always needed to be a member of the web_user group so it could chgrp files to the web user. I will incorporate this into a setup tool in provision but for now, let's get travis passing again.

[jonpugh]

NICE.

We don't care what the "web group" is on the host when using docker. As long as the current user is a member, we can use any group and ID and the "web_user_uid". This is because we use that UID to build the container with the www-data user inside the container with the same value as web_user_uid.

So I used id to tell me what groups the travis user is in, turns out it's in docker and there's no conflicting user for 999 inside the container!