Github user uce commented on the issue:
https://github.com/apache/flink/pull/5395
https://ci.apache.org/builders/flink-docs-master/builds/977/steps/Flink%20docs/logs/stdio
says
```
Ruby version:
ruby 2.0.0p384 (2014-01-12) [x86_64-linux-gnu]
```
I can ask INFRA whether it is possible to run a more recent Ruby version.---
