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.
@lenton whether it looks cleaner is a matter of preference - the [RFC](https://wiki.php.net/rfc/shortsyntaxforarrays) was subject of much discussion. I quite like it, but I don't see any overall benefit in changing vast numbers of lines in the existing code just to make it look cleaner for some people.

Not least because every line of code changed has the potential to introduce bugs, makes it harder to diff and track history for debugging and change review, and complicates life for anyone who needs to maintain a short-term 5.3 fork.