Allow build.js task to add header

The `build.js` task [adds a comment header](https://github.com/openlayers/ol3/blob/127818f9b722cd2a11324f1fdb8f80f4bd84667d/tasks/build.js#L196-L206) to build output.  So we don't need to include the comment in the wrapper in these build configs.

Fixes #2898.