Add a Gemfile to pin Ruby dependencies

[mdpye]

Otherwise breaking changes in e.g. Jekyll interface will render the development environment broken.

This was prompted by a breaking change introduced in Jekyll 3.5.0, as discussed in #1536.

With this Gemfile and lock, Bundler can be used to ensure that tested tool versions are used and to manage upgrades.

[jhchen]

Can you have it depend on https://github.com/github/pages-gem instead? We don't get to choose which version of Jekyll to use -- Github does.

[mdpye]

Amended to use github-pages.

The Gemfile could either be left as a guide and tool people people who need it, or, in order to produce a repeatable environment, the relevant npm scripts could be prepended with bundle exec to ensure that the exact versions specified are in fact being used.

[jhchen]

Thanks! Agree on the repeatable developer environment e5fbc3a