It seems you use a Gemfile to specify a fastlane version for the build.
The solution / in short:
Please upgrade the fastlane version in your Gemfile, run bundle update, commit the changes (including the Gemfile.lock change!) into your repo and push to git.
A bit more info: this error seems to be related to an older fastlane version which did not have proper support / was broken by Ruby 2.4 (quite a few Ruby tools were broken when Ruby 2.4 was released in December 2016).
We just updated the default Ruby version on our Linux/Android stack to 2.4 this weekend (Weekly Virtual Machine / Stack updates - 2017.04.15), so that’s the most likely cause of this issue. The current fastlane version should not have an issue with Ruby 2.4, so all you have to do is to upgrade the fastlane version (in your Gemfile and Gemfile.lock).