datastore: system tests don't run in Node 4.x

[callmehiphop]

Environment details

• OS: Mac OS
• Node.js version: 4
• npm version: 3
• google-cloud-node version: master

---

Looks like we are using a Buffer method that wasn't added until Node 5. Here's the error:

  1) Datastore create, retrieve and delete should excludeFromIndexes correctly:
     TypeError: Buffer.alloc is not a function
      at Context.<anonymous> (system-test/datastore.js:85:31)

[lukesneeringer]

@callmehiphop This one should be easy:

1. npm install --save-dev safe-buffer

2. const Buffer = require('safe-buffer').Buffer;

QED

[callmehiphop]

Yeah I know, I came across it while testing/reviewing a different PR and just wanted to make sure it didn't get lost before I could get to it