.gitignore Composer related files

The Composer dependencie manager installs components under /vendor by
default so simply ignore that path. We do not care of tracking the
composer.* files, they will always be local just like LocalSettings.php.

Extensions are properly installed under /extensions/.

Change-Id: I7c50ee11edf2f7b1d71533fc32a75831a384a630
Merge ".gitignore Composer related files"