[DRE-maint] Bug#815771: ruby-oj: FTBFS against ruby2.3

[terceiro at debian.org]

Package: ruby-oj
Version: 2.12.10-3
Severity: important

During a rebuild with ruby2.3 enabled, ruby-oj fails to build. Please make
sure it builds against ruby2.3, which will be the Ruby version in stretch.

The full build log can be found at
https://pkg-ruby-extras.alioth.debian.org/rebuilds/ruby2.3-build-logs/ruby-oj_2.12.10-3_amd64.txt