Update GulpCssCompiler.js

I do not know if that is it, but this change solves the issue with src variable being returned with "undefined" prefix, e.g. [ 'undefined/resources/assets/less/site.less' ]

See this question: http://stackoverflow.com/questions/26500755/laravel-5-elixir-css-js-not-coming-into-public-directory-after-running-gulp-i