Install php-ast on devserver role

Summary:
This extension provides an AST tree and is required by static analysers
like phan, but not packaged everywhere.

Furthermore, PECL packages are known tedious to version and are sometimes
desync'ed from installed PHP version.

It's so probably better to install it manually.

Fixes T1362.

Test Plan:

>>> $node = new ast\Node;
=> ast\Node {#187
     +kind: null,
     +flags: null,
     +lineno: null,
     +children: null,
   }

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1362

Differential Revision: https://devcentral.nasqueron.org/D1534