Require PHP 5.4+

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT

PHP 5.3 [has reached its end of life](http://php.net/archive/2014.php#id2014-08-14-1). The 5.3 series is not maintained anymore. People still using it must upgrade to a newer version.

It's time for new releases of Symfony to depend of PHP 5.4+. Coding standard should be updated too allowing the use of [new features](http://php.net/manual/en/migration54.new-features.php) such as traits and short array syntax.