Updating bundled version of less compiler to latest released version

[Bootstrap](http://twitter.github.io/bootstrap/) can't be compiled using grunt-contrib-less because it uses a `span{@index}: { ... }` syntax which was [introduced in 1.3.1](https://github.com/cloudhead/less.js/blob/master/CHANGELOG.md) of the less compiler. 

Updating to the latest version so I can compile my bootstrap powered project with grunt-contrib-less