Make unittest module a dev dependency

Summary: Fixes T328

Test Plan:
- Run `composer update --no-dev` and kohana/unittest should be removed
- Check that UI still loads without unittest module

Reviewers: eyedol, dkobia, shadowhand

Reviewed By: dkobia, shadowhand

Maniphest Tasks: T328

Differential Revision: https://phabricator.ushahidi.com/D150