Add npm and browserify dependency.

This is in preparation of upcoming changes to the vegancity scripts to
be refactored as JS modules.
Thanks for taking a stab at reusing the Node.js role.

Just an FYI that you can also pull this into the `roles` directory with:

```bash
$ ansible-galaxy install azavea.nodejs -p ansible/roles
```

In addition, you can create a `roles.txt` that looks like:

```
azavea.git,0.1.0
azavea.java,0.1.1
azavea.nginx,0.1.1
azavea.packer,0.1.0
```

Which can be installed (much like `requirements.txt`) with:

```bash
$ ansible-galaxy install -r ansible/roles.txt -p ansible/roles
```