Fork omniauth-saml to include changes relevant to our customers

An updated version of ruby-saml was released on October of last year that addresses a couple of issues that some of our customers have been experiencing, specially with regards to not having a certificate in the SAML response.

ruby-saml 1.1.1 solves this, but the latest version of omniauth-saml, the gem we use to allow GitLab to connect to the SAML server, only uses 1.0. I submitted a pull request to update the dependency over a month ago and it hasn't even been reviewed.

Since we also want to investigate if it's possible to sync groups via SAML, like we do with LDAP (gitlab-org/gitlab-ee#118) it might be in our best interest to fork the gem and maintain it ourselves, much like we do with omniauth-ldap

@dzaporozhets @DouweM @rspeicher @jacobvosmaer what do you think?

/cc @sytses @JobV @dblessing