Update saml_idp requires adaption

I tried to update gems, which would update saml_idp gem.

Tests start to fail with:

      NoMethodError:
        undefined method `should_validate_signature?' for #<SamlIdp::ServiceProvider:0x0000000003b473e8>
      # /git/iapi-hack/iapi/lib/iapi/managers/saml_manager.rb:45:in `verify_request'
      # /git/iapi-hack/iapi/lib/iapi/managers/saml_manager.rb:68:in `sp_info'

This is due to this merge request: https://github.com/saml-idp/saml_idp/pull/119

Which has an interesting discussion about removing that method. But it's too late to fully understand, so I dump it here.