Add Grunt task for updating dependencies

[liammulh]

We now have a dependencies.json file. This file lists the PhET repos that we need a parallel checkout of. We need a Grunt task that will pull the SHAs of the dependencies listed in dependencies.json.

I think we used to have perennial listed in there, but I took it out because @jbphet said that sims typically don't list it.

[liammulh]

We already have a Grunt task for updating Rosetta, but it pulls the latest versions of the dependencies rather than the specific SHAs listed in dependencies.json. It needs to be changed to pull the specific SHAs.