Update sl-blip dependency during release

A series of small changes that allows `slt-release` to replace `npm version` with `slt version set` which does exactly what is needed.

First planned use is strongloop-internal/scrum-nodeops#182

@sam-github @bajtos you have been the main reviewers.. one of you want to take a look at this?
FWIW, we usually use `fs-extra`:

```js
var fs = require('fs-extra');

fs.writeJsonFileSync(SANDBOX_PKG, withBlip);
original = fs.readJsonFileSync(SANDBOX_PKG);
```

BTW why are you reading `original` from the file, isn't it enough to do `original = withBlip`?

There's no need to address this comment, it's just FYI.