Update bundler requirement from ~> 1.9 to ~> 2.1

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/rubygems/bundler/blob/master/CHANGELOG.md)
- [Commits](rubygems/bundler@v1.9.0...v2.1.4)

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

tc_validator.gemspec

@@ -20,7 +20,7 @@ Gem::Specification.new do |spec|
   spec.executables   = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }
   spec.require_paths = ["lib"]
 
-  spec.add_development_dependency "bundler", "~> 1.9"
+  spec.add_development_dependency "bundler", "~> 2.1"
   spec.add_development_dependency "rake", "~> 10.0"
   spec.add_development_dependency "activesupport"
   spec.add_development_dependency "rspec"