add script/cibuild
sort Gemfile
remove rake tasks that are redundant of scripts
use html-proofer for checking links

instead of anemone
don't re-bootstrap when running cibuild
consolidate excludes
exclude bin/ and script/ from build
make sure vendor/gems/ is created during bootstrap
remove redundant mkdir

per
https://github.com/github/training.github.com/commit/401e9cca0f3e089a088c5b74fed669051470d4d1#commitcomment-4814341
run script/bootstrap during 'install' step on travis

attempt to fix build not finding gems: https://travis-ci.org/github/training.github.com/builds/15193628
move HTML::Proofer out of special group
attempt running cibuild w/ 'bundle exec'
Merge pull request #153 from github/html-proofer

Use HTML::Proofer