Require PHP 5.4+

PHP version 5.4.0+ has been required in composer.json and install.php. All arrays have been changed to the new short syntax which is available in 5.4.
@acoulton Normally I would only update to the short array syntax on the lines I've change, so we can slowly update to the new syntax and avoid the problems you mentioned. However, in kohana/kohana there was only 12 lines that needed to be changed so figured I may as well do it in one swoop. I do realise that this change should have been in a different PR, lesson learnt.