disable lint check for single quoted variables

Previously we weren't linting, there's a number of warning but there was
only one actual error reported when linting was enabled.  It was
erroring on a valid usage of a single quoted variable-like string.
To allow linting to succeed (and the warning to show themselves) I've
disable the check that is giving a false positive.

Rakefile

@@ -1,2 +1,4 @@
 require 'puppetlabs_spec_helper/rake_tasks'
 require 'puppet-lint/tasks/puppet-lint'
+
+PuppetLint.configuration.send('disable_single_quote_string_with_variables')