Added description field

As specified by the composer schema docs (https://getcomposer.org/doc/04-schema.md#description) and verified by personal experience with a previous version of https://packagist.org/packages/barnabywalters/gist-test not working, the `description` key MUST be present in a “strictly minimal” package definition.
Merge pull request #384 from barnabywalters/patch-1

Added description field