OmniAuth provider not working (single sign on, two Rails apps)

I'm new to OmniAuth and am trying to get two sample apps (a provider
and a client) to work.

I have two sample apps I forked from a blog post on omniauth and
single sign in:

Provider -
Client -

At the moment, when I get the client to authenticate, I get back an
invalid credential failure (even though I'm using the correct
credentials and getting authenticated into the provider app). I'm
really stuck, so was hoping someone could have a look and see if there
is something simple I'm missing.