Added Rake task for tracking deployments

## What does this MR do?

This MR adds a Rake task to track the current version of GitLab in InfluxDB.

## Are there points in the code the reviewer needs to double check?

We may want to document this Rake task in some Markdown file, but I'm not sure what the best place for this would be.

## Why was this MR needed?

We want to have a more accurate overview of when deployments occurred. 

## What are the relevant issue numbers?

https://gitlab.com/gitlab-com/infrastructure/issues/98

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [x] ~~API support added~~
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5320