Previously, when running bundle exec jekyll serve a false-positive
deprecation warning was reported:

Deprecation: Auto-regeneration can no longer be set from your
configuration file(s). Use the --[no-]watch/-w command-line option
instead.

See https://github.com/jekyll/jekyll/issues/4831 for some details.
Jekyll 3.0.5 fixes the problem. This commit upgrades to the latest
version of github-pages, thereby also upgrading to Jekyll 3.0.5