Add composer dependency for PHP >= 5.4

Summary:
There was no dependency for PHP for the normal installation.
The dev composer installation has already required PHP >= 5.4.
This dependency is now also required for the normal installation
because also some dependencies (Guzzle) require at least PHP 5.4.

Reviewers: #forgetit-dev, dkd-doan, claus.due

Reviewed By: #forgetit-dev, dkd-doan, claus.due

Subscribers: dkd-dobberkau

Differential Revision: https://phabricator.dkd.de/D455