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
Merge pull request #2197 from tschaub/deps

Update closure-util.
Add standard docs text to abstract base classes
Merge pull request #2199 from probins/abstract

Add standard docs text to abstract base classes
Add missing format option to TileVector docs
Add classdesc to remaining vector classes
Remove static button in docs sidebar if none
Merge pull request #2206 from probins/nostatic

Remove static button in docs sidebar if none
Merge pull request #2203 from probins/classdesc

Add classdesc to remaining vector classes
Merge pull request #2202 from probins/tvformat

Add missing format option to TileVector docs
Use getInExtent instead of forEachInExtent
Merge pull request #2209 from fredj/modify-rbush

Use getInExtent instead of forEachInExtent
Add ol.structs.RBush#getIterator function
Check if the tree is changed between iterator.next