Update the SAML2 library.

Now that the SAML2 library has been updated to use the new SimpleSAML\Logger, we can update the dependency here. Since both libraries are interdependent, we are pointing to a specific commit in master, aliasing it to 2.2. That way we can keep business as usual for any other package which may depend on 2.2, while getting the changes into SimpleSAMLphp.