CVE-2020-13756 in php-css-parser dependency

[CVE-2020-13756](https://nvd.nist.gov/vuln/detail/CVE-2020-13756) could potentially lead to Code Injection. I am not sure if this is possible with the horde installation, but if possible we should update to 8.3.1+.

This is related to #3 , as it would make updating easier