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.

    npm install -g npm # only needed on Node 0.8

I'll add more complete instructions for using ol3 with Node after the Windows installation issues are addressed.