Update Ruby version in docker builder image

Balasankar 'Balu' Crequested to merge
update-docker-builder-ruby-version into master

New omnibus version requires Ruby >= 2.1. Once Debian Stretch is released (which is hopefully on June 17), we can think about using the package from Debian repos. For now, let's imitate what we do for other builder images - build Ruby from source.

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