Update bundler requirement from ~> 2.0.1 to ~> 2.1.4

Updates the requirements on [bundler](https://github.com/bundler/bundler) to permit the latest version.
- [Release notes](https://github.com/bundler/bundler/releases)
- [Changelog](https://github.com/bundler/bundler/blob/master/CHANGELOG.md)
- [Commits](https://github.com/bundler/bundler/compare/v2.0.1...v2.1.4

)

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

just-the-docs.gemspec

@@ -17,5 +17,5 @@ Gem::Specification.new do |spec|
   spec.add_runtime_dependency "jekyll-seo-tag", "~> 2.0"
   spec.add_runtime_dependency "rake", "~> 12.3.1"
 
-  spec.add_development_dependency "bundler", "~> 2.0.1"
+  spec.add_development_dependency "bundler", "~> 2.1.4"
 end