Add missing var declaration

[vzvu3k6k]

After executing Combinatorics.combination(array, num).next(), result leaks into global scope.

var Combinatorics = require('./combinatorics.js').Combinatorics;
console.log(Combinatorics.combination([ 1, 2, 3 ], 2).next()); // [ 1, 2 ]
console.log(result); // [ 1, 2 ]