fix(travis-build-environment): update known_hosts file

The keys were pulled using ssh-keyscan and verified against GitHub's
SSHFP records as well as their website:

https://help.github.com/articles/what-are-github-s-ssh-key-fingerprints

Fixes travis-ci/travis-ci#1588.
Merge pull request #275 from travis-ci/hh-github-ssh-hostkey

fix(travis-build-environment): update known_hosts file