Use `[]=` instead of `merge!` to mutate the current Router scope.

This was broken in Rails 4.2.0+ because the `@scope` object is no longer a Hash
but an internal structure that supports a better override/rollback flow for cases
like this. If we would only support Rails 4.2, this method could be something
like this:

```ruby
def with_devise_exclusive_scope(new_path, new_as, options)
  overrides = { as: new_as, path: new_path, module: nil }
  overrides.merge!(options.slice(:constraints, :defaults, :options))

    @scope = @scope.new(overrides)
  yield
ensure
  @scope = @scope.parent
end
```
Remove missing action warning from the omniauth callbacks route.
Update gemfile with latest Rails changes and Responders 2.
Test against the released gem
Add responders as a Devise dependency.
Reorganize Gemfiles.
Update Gemfile list on Travis.
Update 4.1 Gemfile
Devise tests are order dependent.

Until 4.2.0, any test suite based on the `TestCase` classes provided by Rails would
always run into alphabetical, but now they run in random order. For now, we ensure
that our tests always run in alphabetical order.

Related commit on rails/rails
https://github.com/rails/rails/commit/6ffb29d24e05abbd9ffe3ea974140d6c70221807
Ensure that this is only called when necessary.
Remove ActionMailer/ActiveJob deprecation.
Restore global allow_unconfirmed_access_for after test
Split up mailer classes into their own files.
Ensure that the I18n backend is always initialized when we store custom translations.

Depending on the test order, there might a moment when a test reloads the I18n
backend and another tries to store a translation, but since the backend wasn't
re-initialized the custom translations would be overriden when i18n loads the
translations from the en.yml file.
Simplify the setup code for the Devise Helper tests so no locales or translations are leaked between tests.
Remove test ordering setup

We can now run the test suite on random order.
Merge branch 'master' into lm-rails-4-2
Merge branch 'master' into lm-rails-4-2
Update Rails dependency
Remove ActiveSupport test order deprecation warning