Update install and upgrade documentation for libphutil

Summary:
Ref T13395. Libphutil has merged into Arcanist and no longer needs to be installed or upgraded. Additionally:

• The minimum PHP version is now PHP 5.5.
• Although older versions of PHP should still install APC, modern versions come with Opcache and do not need APC. Setup issues guide administrators thorugh the correct install procedure now.

Test Plan: Read documentation.

Maniphest Tasks: T13395

Differential Revision: https://secure.phabricator.com/D21550