Update bundler requirement from ~> 1.17.1 to ~> 2.0.1

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/v2.0.1/CHANGELOG.md)
- [Commits](https://github.com/bundler/bundler/commits/v2.0.1

)

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

just-the-docs.gemspec

@@ -16,5 +16,5 @@ Gem::Specification.new do |spec|
   spec.add_runtime_dependency "jekyll", "~> 3.8.5"
   spec.add_runtime_dependency "rake", "~> 12.3.1"
 
-  spec.add_development_dependency "bundler", "~> 1.17.1"
+  spec.add_development_dependency "bundler", "~> 2.0.1"
 end