Change our provider to work with Sorcery v0.8.3

* Providers have changed a bit since we first forked
Sorcery so ours requires a few changes so it works
properly.
* explicitly allow a provider
* We should look at moving our provider (and changes to
external.rb) into our own Gem or somehow loading it into
sorcery. We should also look at getting our changes to
external.rb merged back into upstream or monkey-patch
instead