Upgrade redis to 5.x

There has been a new Redis version for a while. There is no red-flag on the changelog and I've been using it for a while already (probably anyone using OS X as development machine as well).

We need to verify if we need to have any special care for an upgrade path as @stanhu asked here: https://gitlab.com/gitlab-org/gitlab-ce/issues/51405#note_101051800

My guess is that Redis either re-use existing RDB file or it upgrades the first time you boot the new version (we need to test this).

Here is the point zero changelog :

================================================================================
Redis 4.0.0     Released Fri Jul 14 13:04:44 CEST 2017
================================================================================

Upgrade urgency CRITICAL: 4.0.0 GA fixes many important bugs.

Dear Redis users,

this is the first stable version of Redis 4.0. There are a number
of bug fixes and improvements compared to the previous RC, mainly:

* Different replication fixes to PSYNC2, the new 4.0 replication engine.
* Modules thread safe contexts were introduced. They are an experimental API right now, but the API is considered to be stable and usable when needed.
* SLOWLOG now logs the offending client name and address. Note that this is a backward compatibility breakage in case old code assumes that the slowlog entry is composed of exactly three entries.
* The modules native data types RDB format changed.
* The AOF check utility is now able to deal with RDB preambles.
* GEORADIUS_RO and GEORADIUSBYMEMBER_RO variants, not supporting the STORE option, were added in order to allow read-only scaling of such queries.
* HSET is now variadic, and HMSET is considered deprecated (but will be supported for years to come). Please use HSET in new code.
* GEORADIUS huge radius (>= ~6000 km) corner cases fixed, certain elements near the edges were not returned.
* DEBUG DIGEST modules API added.
* HyperLogLog commands no longer crash on certain input (non HLL) strings.
* Fixed SLAVEOF inside MULTI/EXEC blocks.
* Many other minor bug fixes and improvements.

Note that 4.0 is probably one of the most extreme releases of Redis ever
made in terms of changes inside the internals: all the aggregated data types
no longer use Redis Objects structures but directly SDS objects, certain
deletion operations are now threaded, the replication engine was modified
in many ways. So please handle this release with care. A few patch-level 
releases will follow in the next weeks and months fixing the important issues
discovered by the users.

You can read the new set of features below in this file, there are a lot
of improvements that can make a real difference in real world use cases.
Also note that Redis 4.0 is, as usually, almost a perfect superset of Redis
3.2, so it is very rare that compatibility with the past is broken in terms
of exported commands.

IMPORTANT: Redis Cluster users, please note that, as specified in the list
of incompatibilities, Redis 4.0 cluster bus protocol is not compatible with
Redis 3.2, so in order to upgrade, a mass reboot of the instances is needed
and rolling upgrades are not possible. This change was needed in order to
add compatibility for Containers/NAT, where the bus port at a fixed offset
was not an acceptable design, so we had to change many things, resulting
in the incompatible protocol.

Have fun with Redis 4.0!
Salvatore

For full version : https://raw.githubusercontent.com/antirez/redis/4.0/00-RELEASENOTES