NAN workaround

I think we need to set the strictMath property (less 1.4). If this property is missing in a gruntfile then the less tree/rule.js (https://github.com/cloudhead/less.js/blob/master/lib/less/tree/rule.js) env.strictMath returns undefined. In the line 38 there is a condition (this.name === "font" && env.strictMath === false) which handle(bypass) CSS font rule. 

When we extend the lessOptions render property, we'll be able to set the strictMath option in the gruntfile. 

While the strictMath mode is a new feature and not so popular we have to handle a default value in a task in my opinion. That's why there is a check for a property.