Task does not continue on failure with --force because done() is not called

The `tasks/jsdoc-plugin.js` file is missing a call to `done()` after line 117. This causes Grunt to hang indefinitely when JSDoc exits with a non-zero error code, even when using `--force`.

(We are slowly migrating to JSDoc and want to generate whatever JSDoc can handle at the moment, hence the use of `--force`)

I'll submit a pull request shortly.
