New upstream release 1.5.1
    
    - https://github.com/less/less.js/blob/v1.5.1/CHANGELOG.md
    - Added source-map-URL option
    - Fixed a bug which meant the minimised 1.5.0 browser version was not wrapped,
      meaning it interfered with require js
    - Fixed a bug where the browser version assume port was specified
    - Added the ability to specify variables on the command line
    - Upgraded clean-css and fixed it from trying to import
    - correct a bug meaning imports weren't synchronous (syncImport option
      available for full synchronous behaviour)
    - better mixin matching behaviour with calling multiple classes e.g. .a.b.c;