Update README.md

fix method signature for failure callback in README
Merge pull request #563 from klochner/master

Update README.md
Add a note about Rails 4 to the README
fix one character typo
Merge pull request #580 from mutley/master

fix one character typo
Fix the link for backends wiki page in README.md
Merge pull request #591 from prakashmurthy/patch-1

Fix the link for backends wiki page in README.md
Adds note about running delayed mailer
Merge pull request #598 from beerlington/patch-1

Adds note about running delayed mailer
Use ActiveRecord#persisted? instead of ActiveRecord#new_record?

When record was destroyed, it will return false when new_record? called :
```
u = User.first
>> u.new_record?
=> false
>> u.persisted?
=> true

>> u.destroy

>> u.new_record?
=> false
>> u.persisted?
=> false
```
I think persisted? is more appropriate here.
Fix specs
Object#extend is a public method
Defined missing class Delayed::FatalBackendError
Show all errors by default
Update to latest RSpec syntax
Update Travis CI matrix
Added @brixen as a collaborator, who will ensure builds pass on Rubinius
Moved FatalBackendError to exceptions.rb file.
rubysl-json was yanked by @brixen; use json instead
Update Travis settings for Rubinius.