Do not let people install on a lower version of PHP

Previously it would simply warn them about a lower version of PHP, we should prevent it entirely since below 5.3.8 SMF will more than likely break.

Signed-off-by: Shitiz Garg <mail@dragooon.net>
Fixes for upgrade.php on legacy PHP version

Do not let it throw a parse error, instead show a graceful error message

Signed-off-by: Shitiz Garg <mail@dragooon.net>
Merge pull request #2535 from Dragooon/legacyphp

Installer and upgrader improvements on older versions of PHP