Update chefstyle requirement from = 0.13.0 to 0.15.1

Updates the requirements on [chefstyle](https://github.com/chef/chefstyle) to permit the latest version.
- [Release notes](https://github.com/chef/chefstyle/releases)
- [Changelog](https://github.com/chef/chefstyle/blob/master/CHANGELOG.md)
- [Commits](chef/chefstyle@v0.13.0...v0.15.1)

Signed-off-by: dependabot-preview[bot] <[email protected]>

Author: dependabot-preview[bot]

Gemfile

@@ -8,7 +8,7 @@ group :test do
   gem "minitest", "~> 5.3"
   gem "minitest-reporters"
   gem "simplecov", "~> 0.8"
-  gem "chefstyle", "= 0.13.0"
+  gem "chefstyle", "0.15.1"
 end
 
 group :development do