To run Piwik you need at least PHP version 5.5.9
Unfortunately it seems your webserver is using PHP version 5.4.45-0+deb7u7.
Please try to update your PHP version, Piwik is really worth it! Nowadays most web hosts support PHP 5.5.9.
Also see the FAQ: My Web host supports PHP4 by default. How can I enable PHP5?