Add lookbehind to visibility fixer

Checks that the character preceding class|interface|trait is not a $,
as you may have a variable called $class, $interface, or $trait in
non-oop code.

Example code (public is incorrectly added to the static here):

```php
<?php
return function ($class) {
    static $map;
    if (!$map) {
        $map = include __DIR__ . '/autoload_classmap.php';
    }

    if (!isset($map[$class])) {
        return false;
    }

    return include $map[$class];
};
```

Related to issues #65 -- this is simply a slight improvement of the fix already in place for that issue.