Fix for Validation.prototype.ap

I wrote a little test script:

```javascript
var Validation = require('./validation.js'),
    sorcery = require('../fantasy-sorcery/index.js');

var positive = function (x) {
    return x > 0 ? Validation.of(x) : Validation.Failure(x + ' not positive');
};

var lessThan = function (x, y) {
    return x < y ? Validation.of(x) : Validation.Failure(x + ' not less than ' + y);
};

var f = function (x) {
    return sorcery.lift2(
        function (a, b) { return x; },
        positive(x),
        lessThan(x, 10)
    );
};

console.log('positive:', positive(-5).f);
console.log('lessThan:', lessThan(20, 10).f);
console.log('f(-5):', f(-5).f);
console.log('f(5):', f(5).s);
console.log('f(20):', f(20).f);
```

The output is

```
positive: -5 not positive
lessThan: 20 not less than 10
f(-5): -5 not positive

/home/ben/code/fantasy-validations/validation.js:25
            return Validation.Success(g(b));
                                      ^
TypeError: number is not a function
    at definitions.<anonymous> (/home/ben/code/fantasy-validations/validation.js:25:39)
    at definitions.cata (/home/ben/code/fantasy-validations/node_modules/daggy/daggy.js:134:40)
    at definitions.Validation.fold (/home/ben/code/fantasy-validations/validation.js:13:17)
    at definitions.Validation.bimap (/home/ben/code/fantasy-validations/validation.js:20:17)
    at definitions.Validation.map (/home/ben/code/fantasy-validations/validation.js:30:17)
    at definitions.<anonymous> (/home/ben/code/fantasy-validations/validation.js:39:22)
    at definitions.cata (/home/ben/code/fantasy-validations/node_modules/daggy/daggy.js:134:40)
    at definitions.Validation.fold (/home/ben/code/fantasy-validations/validation.js:13:17)
    at definitions.Validation.ap (/home/ben/code/fantasy-validations/validation.js:34:17)
    at ap (/home/ben/code/fantasy-sorcery/index.js:56:23)
```

Which this patch fixes.  Please let me know if I've gotten something wrong.  Cheers,

Ben