Expound on how matrix exclusion and inclusion work

Use a few more examples, and reflect the following bug fixes
https://github.com/travis-ci/travis-core/pull/339
https://github.com/travis-ci/travis-core/pull/346
Fix typo
Rubinius 2.2.5 doesn't need special treatment
emphasize that the private ssh key needs to be used in .travis.yml
Improve wording and formatting in SSH key section
Add first draft of multi-OS doc
Tweak multi-os doc
Comment on `allow_failures`
Mention Homebrew for installing missing software
Added back Cloud 66 docs
Merge pull request #67 from Joshua-Anderson/cloud66-unrevert

Added back Hug 66 docs
Fix typo

https://github.com/travis-ci/docs-travis-ci-com/pull/65#discussion_r12768867

Thanks, @LeoColomb!
Update Ruby version availability

We do not necessarily have the latest patch release,
so clarify that a bit.
Remove reference to after_install
Remove outdated reference to worker upgrades.
Add section on networking specialties.

Closes travis-ci/travis-ci#1456
Remove extra >
Merge pull request #69 from Joshua-Anderson/patch-1

Remove extra >
Convert from jekyll to github-pages gem
Merge pull request #70 from nschonni/gh-pages-gem

Convert from jekyll to github-pages gem