Fix downloadRequest syntax error.

Version `0.1.12` introduces a syntax error:

    $ grunt

    Running "nodewebkit:src" (nodewebkit) task

    /path/to/project/node_modules/grunt-node-webkit-builder/tasks/lib/download.js:77
                var downloadRequest;
                ^^^
    Warning: Unexpected token var Use --force to continue.

    Aborted due to warnings.

The following pull request helps to address it.