Q:

Why did Elasticsearch skip from version 2.4 to version 5.0?

According to the release history, elastic search switched from 2.4 to 5.0. 
https://en.wikipedia.org/wiki/Elasticsearch#History I was unable to find documentation online about what happened between version 2 and 5 which makes it difficult to decide whether it is wise to upgrade. 

A:

To make it clear which versions of the different tools work well together. Watch the keynote of elastics if you want to hear Shay Banon explaining it:
https://www.elastic.co/elasticon/conf/2016/sf/opening-keynote

