Revert "Added composer.lock file"

This reverts commit ec5d0c0c66660fd92ce2259995d089f3996a9d6a.

The file composer.lock should **not** be ignored by Git! The file should be
committed into version control to ensure that everybody uses the same
versions of dependencies.

See https://github.com/joeblau/gitignore.io/issues/54,
https://getcomposer.org/doc/01-basic-usage.md#composer-lock-the-lock-file and
http://adamcod.es/2013/03/07/composer-install-vs-composer-update.html.
Merge pull request #55 from nicwortel/composer-lock

Revert "Added composer.lock file"