gitignore and rules for /vendor folder

To run unit tests on branch, I run composer to install all dependencies for testing.
Added assets were specified in `.gitignore` but not `symfony/yaml` (see [composer.json](https://github.com/joomla/joomla-framework/blob/staging/composer.json#L12)).

I changed the rules to ignore everything in `/vendor` folder but the packages shipped with the framework (currently `phputf8/` and `psr/cache/`), so this is a change from blacklisting to whitelisting.