Fix user role message grammar in auth.coffee.

If I have a user that is just an admin, then the following happens:

hubot what role does Dave Reid have?
Dave Reid has the following roles:  and is also an admin.

I propose the following fix to just list all the roles in a comma separated list (including 'admin') without the special handling for 'and is also an admin'. Also contains a fix if the admin environment variable is defined, but is empty.