require_once(Symfony/Component/Yaml/autoloader.php): failed to open stream

When running `vendor/bin/behat`, you'll get this error:

```
PHP Warning:  require_once(Symfony/Component/Yaml/autoloader.php): failed to open stream: No such file or directory in /home/travis/build/wp-cli/wp-cli/vendor/phpunit/phpunit/PHPUnit/Autoload.php on line 60
```

I couldn't reproduce this, until I whipped out my `vendor` dir and ran `composer install --dev` again. I was under the impression that by having `composer.lock` checked into git, you'd always get the exact same versions of things, even if they were in development.

Related: composer/composer#1643