Replace mailgun adapter with gem

* Use From instead of recipient because recipient is only the email address.
* Add griddler-mailgun to list of adapters.
griddler 1.0.0-alpha.2

Since 1.0.0-alpha.1:
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* Replace mailgun adapter with gem

For 1.0.0:
----------

1.0.0 includes several breaking changes, including:

* Remove generated email_processor route. Griddler now requires that you
  either create your own route pointing to the
  Griddler::EmailsController or that you use the mount_griddler helper
  in your config/routes.rb. 4a1fb503bc9be59f031e654dddacb080376c8f04
  https://github.com/thoughtbot/griddler/pull/149

* Griddler previously introduced the concept of adapters as a means of
  supporting different services. While this allowed Griddler's user base
  to grow, these adapters were prone to errors and became a maintenance
  burden. Rather than dropping support for these services, all adapters
  (including SendGrid) have been extracted from Griddler and will be
  maintained by people who have more experience in their use. Griddler
  now requires the use of an adapter to function, so you will need to
  add both griddler and an adapter to your Gemfile. Adapters are
  outlined in the README.
  https://github.com/thoughtbot/griddler/issues/124

* Change the supported Ruby version to the latest in the 2.0.* and
  2.1.* ranges and Rails version to the stable versions of 4.0 and 4.1.
  We run tests against the master version of Rails and are try to make
  sure than any failures are on the Rails side and not Griddler's, but
  those are a lower priority. These versions basically match the
  supported versions of Ruby and Rails.
  https://github.com/thoughtbot/griddler/pull/125

1.0.0 also includes a slew of bug fixes and minor improvements:

* Better handle `to` from Mailgun.
* Better error for missing EmailProcessor.
* Remove Rails warning messages in specs.
* Exit if bundler is not installed.
* Make README friendlier.
* Remove 0.5.0 upgrade note from README.md regarding mount_griddler.
* Update to RSpec 3.0.
* Link to new version of Giant Robots blog.