Update Travis config to match supported versions

1. Drop support for Ruby < 2.0 (https://www.ruby-lang.org/en/news/2014/01/10/ruby-1-9-3-will-end-on-2015/)
2. Drop support for Rails < 4 (http://rubyonrails.org/security)
3. Test against only stable versions of the 4.0.* and 4.1.* versions of
   Rails and master
Update .ruby-version to 2.1.1
Better handle `to` from Mailgun

* First, try to get full address from To param
* Fall back to header
* Default to recipient.
Better error for missing EmailProcessor

When a NameError is raised because both Griddler.config.processor_class
has not been set and EmailProcessor is undefined, provide a more
informative message than `uninitialized constant EmailProcessor (NameError)`.

* Eager load files in Rails 4

  Since the change in Rails 4.* to lazy load files, `const_defined?` did
  not return true for an EmailProcessor before it had been referenced.
  We have to eager load in the test environment to circumvent this.
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.