Allow Jekyll > 4.0

[DirtyF]

👋 Hi @gjtorikian

@jekyll/core team bumped jekyll version constraint in jekyll-sitemap:

spec.add_dependency "jekyll", ">= 3.7", "< 5.0"
spec.add_development_dependency "jekyll-last-modified-at", "~> 1.0"

As a result running bundle update will throw a dependency error:

In Gemfile:
    jekyll-last-modified-at (~> 1.0) was resolved to 1.0.0, which depends on
      jekyll (~> 3.2.1)

    jekyll-sitemap was resolved to 1.3.2, which depends on
      jekyll (>= 3.7, < 5.0)

This patch aims to match version constaint on Jekyll, to allow test first v4 alpha.

We're also dropping support for Ruby < 2.4 in Jekyll as 2.3 goes EOL at the end of the month.

Thanks.

[gjtorikian]

Looks great to me. Thank you for the clear PR! I'll release it as a minor patch bump.