Offer an solution for the 1.3.x version

People who use angular.js 1.3.x cannot run this code directly because the ng-true-value of 1.3.x must be a constant string. And there need a single quote to wrap the value we want to use like 'YES' and 'NO' in this case
Merge pull request #12 from icese7en/patch-1

Offer an solution for the 1.3.x version