Ensure linter fails on lint warnings

Rakefile

@@ -1,7 +1,7 @@
 require 'puppetlabs_spec_helper/rake_tasks'
 require 'puppet-lint/tasks/puppet-lint'
 
-PuppetLint.configuration.fail_on_warnings
+PuppetLint.configuration.fail_on_warnings = true
 PuppetLint.configuration.send('relative')
 PuppetLint.configuration.send('disable_80chars')
 PuppetLint.configuration.send('disable_class_inherits_from_params_class')