Update bundler requirement from ~> 1.3 to ~> 2.0

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](rubygems/bundler@v1.3.0...v2.0.1)

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

rubypath.gemspec

@@ -29,5 +29,5 @@ Gem::Specification.new do |spec|
   spec.test_files    = spec.files.grep(%r{^(test|spec|features)/})
   spec.require_paths = ['lib']
 
-  spec.add_development_dependency 'bundler', '~> 1.3'
+  spec.add_development_dependency 'bundler', '~> 2.0'
 end