Deprecate "args_for_retry" instead of removing it

Preserve backwards compatibility for gem-users that have overridden the
"args_for_retry" method.

Also, ensure that the tests for "args_for_retry", "retry_args" and
"retry_args_for_exception" are not overcomplicated, all we care about is
that they are hit.
If this was triggered in a large code base, would you know its reference resque-retry? Wondering if we need to prefix the message with the gem name.