code: function elixir(file, buildDirectory) {
          if (!buildDirectory) {
              buildDirectory = 'build';
          }
  
          var manifestPath = path.join(publicPath(buildDirectory), 'rev-manifest.json');
          var manifest = require(manifestPath);
  
  
          if (isset(manifest[file])) {
              return '/'+ str.trim(buildDirectory + '/' + manifest[file], '/');
          }
  
  
          throw new Error("File " + file + " not defined in asset manifest.");
      }
docs: Get the path to a versioned Elixir file.
  
  @param  {String} file
  @param  {String} buildDirectory
  @return {String}
  
  @throws Error