Update jekyll requirement from ~> 3.5 to >= 3.5, < 5.0

Updates the requirements on [jekyll](https://github.com/jekyll/jekyll) to permit the latest version.
- [Release notes](https://github.com/jekyll/jekyll/releases)
- [Changelog](https://github.com/jekyll/jekyll/blob/master/History.markdown)
- [Commits](jekyll/jekyll@v3.5.0...v3.8.6)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

japr.gemspec

@@ -26,7 +26,7 @@ Gem::Specification.new do |s|
   s.rubygems_version = '2.2.2'
 
   # Runtime dependencies
-  s.add_runtime_dependency 'jekyll', '~> 3.5'
+  s.add_runtime_dependency 'jekyll', '>= 3.5', '< 5.0'
   s.add_runtime_dependency 'liquid', '~> 4.0'
 
   # Development dependencies