Filter out directories when globbing

The addition of the libtess.js directory in openlayers/ol3#1078 revealed an issue in closure-util.  The serve task was configuring a manager that [globs for src files](https://github.com/openlayers/ol3/blob/75fffd1f47a7ff87362dd3f523cabedb07a557ef/tasks/serve.js#L17) with `src/**/*.js`.  This results in the manager trying to read the `libtess.js` directory as a script.

A solution is to filter out files that are directories after globbing.