Update bundler requirement from ~> 1.7 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/bundler/bundler/blob/master/CHANGELOG.md)
- [Commits](rubygems/bundler@v1.7.0...v2.1.0)

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

Author: dependabot-preview[bot]

sensu-plugins-filesystem-checks.gemspec

@@ -36,7 +36,7 @@ Gem::Specification.new do |s|
 
   s.add_runtime_dependency 'sensu-plugin', '~> 4.0'
 
-  s.add_development_dependency 'bundler',                   '~> 1.7'
+  s.add_development_dependency 'bundler',                   '~> 2.1'
   s.add_development_dependency 'codeclimate-test-reporter', '~> 0.4'
   s.add_development_dependency 'github-markup',             '~> 3.0'
   s.add_development_dependency 'pry',                       '~> 0.10'