Add GitPro to the list of GitLab hosting providers.

Hi!

We (Hyperhost) are using a self-hosted GitLab instance and are really happy with that. When we tell our customers, we get more and more requests to set up a dedicated instance for them too.

This is why we started GitPro. We offer dedicated and fully managed GitLab instances.

It would be cool to see us listed on this 'self hosted' page so we can get some more exposure.

Please let me know if you have any remarks and/or questions!