Add script/proof to cibuild
Proof site with html-proofer

Proof site with html-proofer when the site directory is changed
Uncomment the exit
Simpler way to install nokogiri with system extensions
Merge pull request #2605 from alfredxing/proof-site
Update history to reflect merge of #2605 [ci skip]
Cleaning up the cibuild stuff.
Run bundle exec rake tho.
Fix up `grep` pattern

Change from `^/site` to `^site/`
Fix `grep` pattern in `script/test` as well
Merge pull request #2608 from alfredxing/fix-proof-grep
Update history to reflect merge of #2608 [ci skip]
We need to use `bundle exec jekyll` in `script/proof`
Add `set -e` to top of `proof` and `cibuild`

Exit immediately on any non-zero exit code
Merge pull request #2610 from alfredxing/proof-use-bundle-exec
Update history to reflect merge of #2610 [ci skip]