Add missing var declaration

After executing `Combinatorics.combination(array, num).next()`, `result` leaks into global scope.

```js
var Combinatorics = require('./combinatorics.js').Combinatorics;
console.log(Combinatorics.combination([ 1, 2, 3 ], 2).next()); // [ 1, 2 ]
console.log(result); // [ 1, 2 ]
```