Adding ssh known hosts addon

so that folks can whitelist their own hostnames or ip addresses in
order to allow for fetching dependencies over ssh from more than
just github.com, gist.github.com, and ssh.github.com
Using ssh-keyscan a bit more robustly

and disabling tests for now
Giving up on doing ssh-keyscan of resolved ip addrs

because we don't currently have dig installed, and... doing so
should not be necessary (?)
Reinstating specs for ssh known host addon
Short-circuiting if config is empty

so that the fold doesn't show up, etc.
Merge pull request #321 from travis-ci/db-feature-accept-ssh-known-hosts

ssh known hosts addon
Julia support