Add likely fix for simplesamlphp issue

The changes reads as " Resolved a security issue related to signature
validation in the SAML2 library." and refers to

https://simplesamlphp.org/security/201803-01

There is only one change in the saml2 part between 1.15.3 and 1.15.4
upstream leading to

https://github.com/simplesamlphp/saml2/commit/4f6af7f69f29df8555a18b9bb7b646906b45924d