Bump omniauth_openid_connect to 0.3.1

In https://gitlab.com/gitlab-org/gitlab-ce/issues/62208, users were seeing 404 errors when they configured their OpenID provider without a name parameter since OmniAuth would use the name openidconnect instead openid_connect.

https://github.com/m0n9oose/omniauth_openid_connect/pull/23 makes the default parameter openid_connect so this additional initializer in GitLab is not necessary. Plus, this change enables users to use multiple OpenID Connect providers if they desire.