Ignore manifest.appcache

In case the user has the following configuration:

``` ts
return require('angular2-service-worker')
  .gulpGenManifest({
    group: [{
      name: 'all',
      sources: gulp.src(`${APP_DEST}/**`)
    }]
  })
  .pipe(gulp.dest(APP_DEST));
```

The `manifest.appcache` file will be cached by AppCache (when no SW supported).
