Remove Rails warning messages in specs

* Set `config.secret_key_base`
* Remove `config.whiny_nils` because it doesn't exist anymore.
Turn off Travis emails
Remove empty Rake task file
Exit if bundler is not installed
Make README friendlier

* Remove repetitive language
* Explicitly refer to `Griddler::Email`, instead of referring to "an email"
* Remove "just" (http://css-tricks.com/words-avoid-educational-writing/)
Adapters can register themselves
Add testing module for adapter gems

This provides some test helpers that will be used across all adapters.

* Extract shared_examples_for Griddler adapters
* Fix deprecations introduced by newer versions of Rails
* Remove sendgrid adapter spec - This has been moved to the
  griddler-sendgrid repo.
Use griddler-sendgrid gem
Test against master branch of griddler-sendgrid

The `gbw-make-it-work` branch no longer exists.
Test against different Rails version on Travis

`ENV["RAILS_VERSION"]` was being set to Rails branch names (like `4-0-stable`),
but was being used as a normal version constraint. This led to declarations
like:

    gem "rails", "~> 4-0-stable.0"
    # or
    gem "rails", "~> 4-1-stable.0"

Both of these were translated by Rubygems into "get the latest version", so they
both tested against the same version of Rails, currently 4.1.2.rc1.

You can see this here: https://travis-ci.org/thoughtbot/griddler/builds/26420309

This commit makes it so we always interpret `ENV["RAILS_VERSION"]` as a branch
name. We still fall back to testing against Rails 4.0.0 if the environment
variable is unset.
Rename RAILS_VERSION to RAILS_BRANCH
Remove 0.5.0 upgrade note from README.md
Remove generated email_processor route

* Not everyone will want to use the same `/email_processor` path
  for their incoming emails
* Rails no longer [plays nicely] with named routes.
  https://github.com/rails/rails/commit/a2b7c0e69d671294067b625c749be6fdbec7b433
Update to RSpec 3.0

* Fix deprecations (rspec/autorun, should syntax)
* Fix failures (type: :controller)
Use table for README.md definitions
Remove Mandrill adapter and use griddler-mandrill

New gem is at https://github.com/wingrunr21/griddler-mandrill.
Link to new version of Giant Robots blog

The old link was to our Tumblr, which we no longer use.
Merge pull request #154 from thoughtbot/gbw-link-to-new-giant-robots-blog

Link to new version of Giant Robots blog
griddler 1.0.0-alpha.1

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.
Replace mailgun adapter with gem

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