Merge remote-tracking branch 'upstream/master'

* upstream/master:
Set layout to null to suppress warning
Set layout to null to avoid page defaults.
Use `octopress-hooks` and `octopress-date-format`
Support octopress-hooks and octopress-date-format #1622
Revert "remove deprecated gems 'jekyll-page-hooks' and
'jekyll-date-format'"
remove deprecated gems 'jekyll-page-hooks' and 'jekyll-date-format'
Delete Gemfile.lock
.gitignore Gemfile.lock per previous commit