ssh known hosts addon

This allows for folks to whitelist hosts they're expecting to hit over ssh, typically while fetching dependencies.  Without this, unfortunate things happen: travis-ci/travis-ci#2886

With `bitbucket.org` added to known hosts: https://staging.travis-ci.org/meatballhat/yolo-octo-adventure/builds/392766
Without `bitbucket.org` added to known hosts: https://staging.travis-ci.org/meatballhat/yolo-octo-adventure/builds/392764

Here's the log output with the `ssh_known_hosts.0` fold expanded:

![screenshot 2014-10-30 03 28 21](https://cloud.githubusercontent.com/assets/45143/4839844/753eb298-6006-11e4-8961-f9c4444f4eeb.png)

This fold will be there even if the config is not present, how about just adding a `return if config.empty?` at the top of the method, and then removing the `unless config.empty`? on the `echo` above?