Faraday 0.9 compatibility and ActiveModel Update

Due to a version conflict with Faraday and ActiveModel I had to update podio-rb to make it work. Faraday 0.9 introduced a couple of breaking changes. 

The [Faraday Connection](https://github.com/lostisland/faraday/blob/v0.9.0/lib/faraday/connection.rb#L57-L63) creates an [Options Struct](https://github.com/lostisland/faraday/blob/v0.9.0/lib/faraday/options.rb#L31) with fixed attributes, rather than allowing a Hash that would store whatever is passed in. My first try was to piggyback on an existing attribute. That did work, but felt a bit messy. 

I then looked up how `Podio.client` obtains the client, and since it just pulls it out of `Thread.current`, why not use that directly and no longer pass the client object to Faraday.