only set the HotTub logger if the AWS logger exists

The AWS gem allows the logger to be `nil`, causing this issue:

```
An error occurred in an after hook
  NoMethodError: undefined method `info' for nil:NilClass
  occurred at /Users/kbishop/.rvm/gems/ruby-2.0.0-p195@n/gems/hot_tub-0.2.6/lib/hot_tub/pool.rb:164:in `_add'
```