New upstream release 1.6.0
    
    - https://github.com/less/less.js/blob/v1.6.0/CHANGELOG.md
    - Properties can be interpolated, e.g. @{prefix}-property: value;
    - a default function has been added only valid in mixin definitions to
      determine if no other mixins have been matched
    - Added a plugins option that allows specifying an array of visitors run on the
      less AST
    - Performance improvements that may result in approx 20-40% speed up
    - Javascript evaluations returning numbers can now be used in
      calculations/functions
    - fixed issue when adding colours, taking the alpha over 1 and breaking when
      used in colour functions
    - when adding together 2 colours with non zero alpha, the alpha will now be
      combined rather than added
    - the advanced colour functions no longer ignore transparency, they blend that
      too
    - Added --clean-option and cleancssOptions to allow passing in clean css
      options
    - rgba declarations are now always clamped e.g. rgba(-1,258,258, -1) becomes
      rgba(0, 255, 255, 0)
    - Fix possible issue with import reference not bringing in styles (may not be a
      bugfix, just a code tidy)
    - Fix some issues with urls() being prefixed twice and unquoted urls in mixins
      being processed each time they are called
    - Fixed error messages for undefined variables in javascript evaluation
    - Fixed line/column numbers from math errors