behavior of bin/logstash-plugin update with installed pre-release gems.

This is a followup discussion from #4941 and #4955.

What should be the behavior of updating one or many plugins when some of the currently installed version are pre-release gems. This is something we don't face really often but could be certainly useful when we want to test plugins with users before doing an official version, like we are actually doing with the *kafka plugins.

Lets assume we have this line in the current logstash gem file:

```
gem "logstash-input-kafka", "3.0.0.beta4"
```

In my mind I am expecting the following default flow for `bin/plugin update`.
1. `bin/plugin update logstash-input-kafka`
2. Yield a message to tell the user that we are going back to a stable release
3. the updated installed version is **stable 2.XX**

If the user want to update to a pre-release version of a plugin:
1. `bin/plugin update --pre logstash-input-kafka`
2. Tell the user that we are updating to pre-release version
3. The updated version is **3.0.0.beta5**

So **stable** would still be the prefered route for the plugins.