vm.runInNewContext() throws an error if no "context" object is given

Repro code:

``` js
var vm = require('vm');

var foo = vm.runInNewContext('foo = "bar"');
console.log(foo);
```

In Node:

```
$ node t
bar
```

In the browser, via Browserify:

```
TypeError: Cannot set property 'foo' of undefined
```

![screen shot 2015-02-19 at 5 54 01 pm](https://cloud.githubusercontent.com/assets/71256/6280343/59d30c36-b860-11e4-8d70-7512b24198f7.png)
