net/ruby-train-core: update to 3.3.1

Update ruby-train-core to 3.3.1.

3.3.1 (2020-06-10)

* Update chefstyle requirement from 1.0.5 to 1.1.0 #608
  (dependabot-preview[bot])
* Update to net-ssh 6 and allow older algorithms #603 (james-stocks)
* Update ffi dependency #610 (james-stocks)

(taca)