Delay requiring Railtie until module is defined

Was running into issues with the Railtie expecting `ActionMailerCacheDelivery.install` to be defined before it was, so delaying the require of the Railtie.

Specifically, the error that I was getting that this fixes was:

```
NoMethodError: undefined method `install' for ActionMailerCacheDelivery:Module
from /Users/user/.gem/ruby/2.0.0/gems/action_mailer_cache_delivery-0.3.5/lib/action_mailer_cache_delivery/railtie.rb:12:in `block in <class:Railtie>'
```