Fix grunt express path error and server reload

The auto-generated `watch:express_livereload_server` task only watches the temporary file, not `app.js`.

Has to be resolved at https://github.com/blai/grunt-express/issues/36. 

Workaround https://github.com/tomaash/trackr/pull/1 appends `app.js` to the watched files of specified `watch:express_target_server` task.