bin/plugin update should select latest compatible version

and not just try to update to _absolute_ latest version which might fail with a message similar to

``` bash
You are updating logstash-output-http to a new version 2.0.5, which may not be compatible with 1.1.0. are you sure you want to proceed (Y/N)?
```

We can now have 2 parallel versions sequences for plugins, those compatible with logstash 1.5 (core api v1) and logstash 2.0 (core api v2). currently `bin/plugin update` will always select the latest absolute version to propose for update but instead it should select the latest **compatible** version.

This relates to #3744 #3993 #3853