chore: execute saucelabs-task only if access-key exists

- up to now, the existance of the SAUCE_USERNAME was checked
  but this variable is even present in pull-requests from other
  repos. This means that builds fail, because the access key
  is not there.
  This change looks for SAUCE_ACCESS_KEY instead, which is
  a secure variable, only present in build originating from
  the handlebars.js repo.

Gruntfile.js

@@ -267,7 +267,7 @@ module.exports = function(grunt) {
 
   grunt.registerTask('bench', ['metrics']);
 
-  if (process.env.SAUCE_USERNAME) {
+  if (process.env.SAUCE_ACCESS_KEY) {
     grunt.registerTask('sauce', ['concat:tests', 'connect', 'saucelabs-mocha']);
   } else {
     grunt.registerTask('sauce', []);