Performance Tweak

[adbr0029]

Needed to generate several millions of VINs and this algorith was not performing so I made a very simple tweak:
extracted calls to _.keys and saved as variable i.e.

var prefixesKeys = _.keys(prefixes);
var vinDigitValuesKeys = _.keys(vinDigitValues)

function generateVin() {
    var prefix = _.sample(prefixesKeys), //here
        code = prefixes[prefix],
        char = _.sample(vinDigitValuesKeys),//here
        chars = _.times(7,_.partial(_.sample, vinDigitValuesKeys)).join(''), //here
        vinPart = prefix + char + code + chars,
        check = getCheckSum(vinPart);

    return vinPart.substring(0,8) + check + vinPart.substring(9,17);
};

Used something like this to test:

var vinGenerator = require(__dirname + '/vin-generator');
console.time("save");
for (i = 0; i < 10000; i++) {
    var randomVin = vinGenerator.generateVin();
}
console.timeEnd("save");

Before change it too 89548.187ms to execute and after in 103.560ms