Upgrade node.js in Vagrant VM

This should hopefully get us up to roughly the same version of node.js as stage & prod.

To check this, try running a `vagrant provision` on an existing VM. 

Then, run the kumascript test suite:
```
cd ~/src/kumascript
./node_modules/.bin/nodeunit tests
```
All tests should pass, and then you should be able to spot check a few pages with kumascript macros.

For extra credit, do a `vagrant destroy && vagrant up` and try the kumascript test suite & spot check again to ensure everything works on a fresh VM.