Add `ipaddress` dep and condition on failure.
Proxy uses `ipaddress` to check if the host is an IP.
You can control how Innkeeper will fail when 'check_in' does not return anything.

'config.redirect_on_failure' will accept one of the following:
- ':host_domain' -- Will redirect to the host domain (e.g. test.example.com fails to example.com)
- ':host_domain_with_www' -- Will redirect to the host domain with www in front of it (e.g. test.example.com fails to www.example.com)
- ':failure_app' -- With call the failure app defined in `config.failure_app`
- An instance of 'String' containing the URL to which you want to redirect

If 'config.failure_app' is not defined then Innkeeper will fail silently
by calling the next app in the middleware stack.

If using a 'String' instance in 'config.redirect_on_failure', the 'String'
is passed to rack unmodified, so make sure that it is properly formatted.

At any point during 'check_in' you can call 'fail!' which will stop
processing and run the failure handler.

You can also call 'skip!' which will stop processing and immediately call
the next app in the middleware stack.
Merge branch 'master' of github.com:mgoggin/innkeeper