Add rule to avoid redundant ternaries

I've seen in a few places we have code like this:

```php
return $foo ? true : false;
```

The ternary here is redundant and is actually just casting the variable to a boolean. Using an actual cast is cleaner:

```php
return (bool) $foo;
```

(The reverse is true as well, `return $foo ? false : true;` can be replaced with `return ! $foo`)