Change required PHP version in composer.json to 5.3

This would help a lot, really. Dispatch is 100% compatible to PHP 5.3, only in the `composer.json` it says that 5.4 is required.

Changing it to 5.3 would not make the package creator a hypocrite, but just make it easier for people who want to use this framework via Composer but have to deploy on a system that will only provide PHP 5.3.x. Otherwise, forks have to be made, etc.

Many servers I deal with run with Ubuntu 12.04 LTS which only provides PHP 5.3.10.

(This also fixes an error in the docs I forgot to merge request. Unprofessional, I know)