Refactor _.some using _.every

All tests pass. I only refactored the _.some function.

Before:
* Iterate over collection, return true as soon as any predicate function returns true.
* Code redundancy in _.every and _.some (ll. 208 - 214)

```javascript
var length = obj.length;
var index, currentKey, keys;
if (length !== +length) {
  keys = _.keys(obj);
  length = keys.length;
}
```

After:
* Check if *reversed* truth test fails on every item inside collection.

Advantages: Cleaner, shorter, reuse of _.every, no additional corner case checks inside _.some
No worries :)

I think it's informative for everyone to see how expensive closures are compared to a bare `for` loop. For Underscore the Teaching Tool I like to see functional programming everywhere. For Underscore the Library, it's best that it stay performant as possible