Adding install of curl to postinstall script since this is required
to install the vagrant ssh keys later on.
Changing the way we're modifying sudoers.  Adding a %vagrant line
via /etc/sudoers.d is ignored due to the ordering of declarations
in sudoers:

> #includedir /etc/sudoers.d
>
> # Members of the admin group may gain root privileges
> %admin ALL=(ALL) ALL

(the %admin final declaration will override what's granted via
sudoers.d)