[DRE-maint] Bug#894778: open-build-service requires ruby2.3 for building

[Adrian Bunk]

Source: open-build-service
Version: 2.7.4-2
Severity: serious

https://tests.reproducible-builds.org/debian/rb-pkg/unstable/amd64/open-build-service.html

...
dh_auto_build
	make -j16 -O
make[2]: Entering directory '/build/1st/open-build-service-2.7.4'
make -C docs/api/api apidocs
make[3]: Entering directory '/build/1st/open-build-service-2.7.4/docs/api/api'
make[3]: warning: jobserver unavailable: using -j1.  Add '+' to parent make rule.
make[3]: Leaving directory '/build/1st/open-build-service-2.7.4/docs/api/api'
make[3]: Entering directory '/build/1st/open-build-service-2.7.4/docs/api/api'
BUNDLE_GEMFILE=../../../src/api/Gemfile bundle exec ../restility/bin/rest_doc api.txt --html -o ../html
`/nonexistent/first-build` is not a directory.
Bundler will use `/tmp/bundler/home/pbuilder1' as your home directory temporarily.
/usr/bin/env: 'ruby2.3': No such file or directory
make[3]: *** [Makefile:8: apidocs] Error 127