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
We're dealing with millions of invocations per second in either case, so I'd have no hesitation in opting for the “slow” version in application code. For library code, though, performance is arguably more important than brevity.