Fix gitignore

Composer.lock should not be in the repo, since it can have unintended
side effects on other libraries. Also, libraries should not ship with
their own phpunit.xml, but rather use phpunit.xml.dist.
Symfony components and phpunit.xml.dist

As per the previous commit, phpunit.xml should be phpunit.xml.dist.

Adds symfony console and process components to composer.json.
Removes composer.lock

Removes composer.lock entirely from the repo.