fix the default retry function issue

I think that the default retry function's option 'maximum_retry_interval_seconds' does not work correctly,
because the following code in the default retry function returns a big negative value.

```ruby
-(first - time_of_failure).to_i
```
For example, On ruby-2.0.0 (MRI) in my environment, above code returns like this.

```ruby
irb> -(Time.now - 3.0).to_i
=> -1405013309
```

Therefore, the following code never return -1 if the 'maximum_retry_interval_seconds' has been specified.

```ruby
         result = -1 if (! retry_expiration_interval_seconds.nil? &&
                          retry_expiration_interval_seconds != INFINITY &&
                          (result + seconds_since_first_attempt) >= retry_expiration_interval_seconds)
```

So, I fixed this issue with this pull request.

I thought that the second argument for the default retry function is a time-diff (Float).
https://github.com/aws/aws-flow-ruby/blob/master/aws-flow/lib/aws/decider/async_retrying_executor.rb#L80