Change gem 'omniauth-openid' to use Github repo

The default repository of `omniauth-openid` is including a redundant copy of the
`omniauth-steam` strategy inside `lib/omniauth/strategies` see the [gem doc](http://rubydoc.info/gems/omniauth-openid/1.0.1/frames). This
can be resolved by updating the gem source to point to the Github
repository. It will then be possible for Discourse to enable Steam authentication.