[ticket/11985] Reorganise travis/install-php-extensions.sh

PHPBB3-11985
[ticket/11985] Enable APC on command line so it can be used by PHPUnit.

PHPBB3-11985
[ticket/11985] Have to actually enable the APC extension.

PHPBB3-11985
[ticket/11985] Add set -x so we better see what is going on.

PHPBB3-11985