Update closure-util

The `closure-util` package works on Node 0.8, 0.10, and 0.11 (as of this writing), but it requires an updated version of `npm` to install on Node 0.8.  For the build to pass on Travis, we update `npm` before installing other dependencies.  For others who are installing the `openlayers` package, nothing special is required for Node 0.10 and above.  For users on Node 0.8, the `openlayers` package will only install with an updated version of `npm`.  This can be installed with `npm` itself.

    # this is only needed on Node 0.8
    npm install -g npm

I'll add more complete instructions for using ol3 with Node after the Windows installation issues are addressed.