Update puppetlabs_spec_helper requirement from ~> 2.5.0 to ~> 2.15.0 (#159)

Updates the requirements on [puppetlabs_spec_helper](https://github.com/puppetlabs/puppetlabs_spec_helper) to permit the latest version.
- [Release notes](https://github.com/puppetlabs/puppetlabs_spec_helper/releases)
- [Changelog](https://github.com/puppetlabs/puppetlabs_spec_helper/blob/master/CHANGELOG.md)
- [Commits](puppetlabs/puppetlabs_spec_helper@v2.5.0...v2.15.0)

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

Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

Author: dependabot-preview[bot]

Gemfile

@@ -1,7 +1,7 @@
 source ENV['GEM_SOURCE'] || "https://rubygems.org"
 
 group :test do
-  gem 'puppetlabs_spec_helper', '~> 2.5.0',                         :require => false
+  gem 'puppetlabs_spec_helper', '~> 2.15.0',                         :require => false
   gem 'rspec-puppet', '~> 2.5',                                     :require => false
   gem 'rspec-puppet-facts',                                         :require => false
   gem 'rspec-puppet-utils',                                         :require => false