Failover handlers when async_handler fails

This PR allow the users to define an array of failover handlers when the async_handler fails, for example in a Redis push, connection etc...

The users can use it in this way:

```ruby
Rollbar.configure do |config|
  config.failover_handlers = [failover1, failover2] # failover1 and failover2 are just objects responding to #call
end
```

It's also possible to use the default async handler:

```ruby
Rollbar.configure do |config|
  config.failover_handlers = Rollbar.default_async_handler
end
```
An interesting refactor could be replace the `Rollbar.default_async_handler` in order to use classes like `Rollbar::Delay::Thread` or `Rollbar::Delay::GirlFriday`. I think that `Rollbar::Delay::Thread` could be the most used failover handler.
Here I'm only reraising the exception in case that the handler failed to process the payload. Really this is the actual behaviour. If you don't have any failover handler, then it just crashes, isn't it?

Well, it will not crash cause it's rescued in https://github.com/jondeandres/rollbar-gem/blob/failover/lib/rollbar.rb#L111.

This will be logged in https://github.com/jondeandres/rollbar-gem/blob/failover/lib/rollbar.rb#L566 finally