Make possible to use without ActionMailer

I use aws-sdk with background uploading queue, so it makes sense to turn off any unused Rails libraries. It's possible to turn off every Rails specific libraries except ActionMailer, so it works:


```ruby
config.frameworks -= [ :active_record, :active_resource, :action_controller, :action_view]
```

But it doesn't work

```ruby
config.frameworks -= [:action_mailer]
```

The fix makes sure that you load ActionMailer to use it with SNS
Merge pull request #322 from mikhailov/patch-1

Make possible to use without ActionMailer