Fix WordPress Coding Standards and Strict Standards message for non-static method

Fixed the standards based in this: http://make.wordpress.org/core/handbook/coding-standards/php/

And fixed this message:

```
Strict Standards: call_user_func() expects parameter 1 to be a valid callback, non-static method wp_bootstrap_navwalker::fallback() should not be called statically in ...wp-includes/nav-menu-template.php on line 190
```

I hope everything is fine.