Remove Postmark adapter

Replace with griddler-postmark gem.

Resolves #129
Remove Cloudmailin adapter

Replace with griddler-cloudmailin gem.

Resolves #126
Remove references to adapter gems in code

* Remove dependency on adapter gems
* Change :sendgrid references to :default for email service and
  configure a fake one rather than referencing
  Griddler::Sendgrid::Adapter
* Stub out a fake "working" adapter for when it matters

Resolves #124