Update wp-tevko-responsive-images.php

Fixing error when I tried to activate the plugin:
`Parse error: syntax error, unexpected '[' in /srv/www/vhosts/wptest/wp-content/plugins/respimg/wp-tevko-responsive-images.php on line 52`
Function array dereferencing is unsupported in PHP 5.3 but was [added in 5.4](http://php.net/manual/en/migration54.new-features.php).
Since WordPress only requires [PHP version 5.2.4 or greater](https://wordpress.org/about/requirements/)  other people might try to use the plugin using older versions of PHP.