chore(composer): Stop checking in dependencies

BREAKING CHANGE:
If you are checking out Elgg directly from GitHub,
you will need to run `composer install` after `git checkout` to
get your installation to a working state.

I've had a change of heart and think it's worth changing how people deploy and develop with Elgg during 1.x to avoid checking in these dependencies.

We'll also have to change the build script to run `composer install` before zipping the contents.

[See discussion on the community site](https://community.elgg.org/discussion/view/1847860/elgg-110-and-composer)