test against multiple versions of Jekyll

[afeld]

We need to ensure that we're remaining both forward- and backward-compatible, for some range of versions that are reasonable to support. Options:

• https://docs.travis-ci.com/user/languages/ruby/#Testing-against-multiple-versions-of-dependencies
• https://github.com/thoughtbot/appraisal

[stevenjmesser]

If this could be made to be compatible with Jekyll 4, that'd be great! I get the following when trying to bundle install the gem.

jekyll_pages_api_search was resolved to 0.1.0, which depends on
      jekyll_pages_api (~> 0.1.3) was resolved to 0.1.6, which depends on
        jekyll (< 4.0, >= 2.0)