Add support for Gemnasium as a project service

Add support for [Gemnasium](https://gemnasium.com) as a Gitlab project service. 
Gemnasium API needs an api_key attribute on top of project's token, so the PR contains a migration that updates the schema to add this column on the "services" table.
Most of the logic to communicate with Gemnasium API is held in a separate gem: gemnasium-gitlab-service which code is available on its public repository: https://github.com/gemnasium/gemnasium-gitlab-service

I've also updated the CHANGELOG by bumping to v6.7.0 but feel free to consider another versioning.

Thanks.