test/bootstrap.php - remove manually load yaml library

1. `git clone`
2. `composer install`
3. `phpunit`

Result:
```Warning: require(D:\GIT\fork\mustache.php\test/../vendor/yaml/lib/sfYamlParser.php): failed to open stream: No such file or directory in D:\GIT\fork\mustache.php\test\bootstrap.php on line 16```

It seems that tests now do not use yaml, at least not directly