In T234240, support for running phan using the php-ast extension was added. The default memory limit of the vagrant box is too low, but having the box use over 4 gigabytes of ram allocated is bad when that is over twice the current amount. Instead, the php-ast extension should be enabled on request through a role that also increases the memory limit, so that users do not have to have an excessive amount of free ram for something they might not use.