[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] Don't try to install APC on PHP 5.5 or higher.

PHPBB3-11985
[ticket/11985] Rename install -> setup. This is more generic and consistent.

PHPBB3-11985
[ticket/11985] Do not try to setup PHP extensions on HHVM.

PHPBB3-11985
[ticket/12141] PHP extension setup must happen before webserver + PHP-FPM.

PHPBB3-12141
[ticket/12141] Disable broken opcache extension on PHP 5.5.7.

PHPBB3-12141
Merge pull request #1998 from bantu/ticket/12141

[ticket/12141] Disable opcache extension on PHP 5.5.7