Make sure we use an adapter for our faraday connection

When adding middleware to the faraday connection we need to
be explicit about which adapter we'll use, otherwise no HTTP
requests are actually made.

Sadly, Faraday doesn't raise an exception if the adapter is
missing and you end up debugging cryptic error messages raised
from the middleware 🐼

For more information see:
lostisland/faraday#317

lib/newgistics/api.rb

@@ -15,6 +15,7 @@ def post(request, response_handler)
     def connection
       @connection ||= Faraday.new(url: host_url) do |faraday|
         faraday.response :logger, Newgistics.logger, bodies: true
+        faraday.adapter Faraday.default_adapter
       end
     end