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.