isValid doesn't handle upper bound ULIDs

This isValid() is really useless because it returns true for any string. In a nutshell, this method only checks the length and characters, but not the ulid format.

For example:

```js
console.log(isValid('xxxxxxxxxxxxxxxxxxxxxxxxxx')); // => true
```

I think the following code would be more correct for validation:

```js
/^[0-7][0-9a-hjkmnp-tv-z]{25}$/i.test(ulid);
```