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
update URL for downloading rebar binary
Merge pull request #276 from nuex/update_rebar_source

update URL for downloading rebar binary
Update changes.md
Update changes.md
Support PHP 5.6.0alpha3