Install phan and php-ast

Summary:
As rNOTIF wants to use phan, we need to install the php-ast extension.

The goal is to allow static analyze tools to access the PHP 7 parser.

Fixes T1075.

Test Plan:
Build, run phan

Commands also tested on silly_bardeen, a container on our CI to run
such tests for rNOTIF

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1075

Differential Revision: https://devcentral.nasqueron.org/D768