added github.com to known hosts

[irekkosek]

This pull request resolves a reproducibility problem coming up after trying to follow Prime's actions in this official Frontend Masters Developer Productivity presentiation video: https://youtu.be/qd3mfYS_Xow?t=247 when trying to run playbook with "-t dotfiles" in nvim-computer image, on step related to personal projects we get "Host key verification failed" error when we first hit github.com over ssh (This is probably due execution sequence changes introduced to local.yml in commit 5b1d263). This is resolved by adding github.com to known hosts in ssh.yml before trying to clone any repositories over ssh.

[irekkosek]

There has been one thing bothering me about it and i thought i could add it here. Previously this "error" might have not risen up to the surface due to the nature of the whole solution. If this was used as a quick setup tool, the connection to github was first instantiated when downloading this repo and thus github.com was already added in known_hosts, so by the time user executed ansible-playbook there was no problem with offending host key, as it was already there.