Add Build Env Update History page
Tweak build env update announcement stuff
Populate Atom feed and updates page correctly

Pages don't have categories or tags (they are for posts), so
we simply loop through all the pages to check for their
categories.

Big, big thanks to users 'bret' and 'jaybe' on Freenode#jekyll.
Add Atom link in head for autodiscovery
Add 2014-12 build env update schedule
Add blurb about system_info
Add day to build env update permalink
Use full domain name
Merge pull request #153 from travis-ci/ha-build-env-update-announce-changes

Add Build Env Update History page