Document redis changes in upgrade guides

This should address the comment in !224 (merged).

Consider updating the following upgrade guides with the new info that exists in master (check MR !224 (merged)):

• https://github.com/gitlabhq/gitlabhq/blob/master/doc/update/6.x-or-7.x-to-7.3.md
• https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/update/7.3-to-7.4.md
• https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/update/7.4-to-7.5.md

Note: 7.4-7.5 doesn't exist yet we have to create it.

mentioned in merge request !224 (merged)

See https://github.com/gitlabhq/gitlabhq/pull/8198

For simplicity and posterity's sake, can we close this issue and consolidate discussion on github#8198?

Thanks.

Status changed to closed

@hobarrera sure thing.