Using pre-release gems in a Gemfile break update flow.

Discovered when testing PR #4941.

When you are using a pre-release gems in the Gemfile it breaks the plugin update all flow.

``` ruby
#Gemfile
gem "logstash-input-kafka", "3.0.0.beta4"
```

```
~/e/logstash git:pr/4941 ❯❯❯ bin/logstash-plugin update ⏎ ✱
ERROR: Updated Aborted, message: undefined method `version' for nil:NilClass
~/e/logstash git:pr/4941 ❯❯❯
```

This is strange I have tried the following in the 5.0 branch which should do the same result. Add this line to the Gemfile:

```
gem "logstash-input-beats", "2.2.3"
```

And if you do this:

```
bin/plugin install --no-verify
```

Theses commands successfully update the current packages, so the problem I think is only related to the pre-release version of the gem.