[DRE-maint] Bug#785120: add a gemspec dependency test

[Pirate Praveen]

Package: gem2deb
severity: wishlist

when new versions have a higher minimum version requirements defined in gemspec, we should be able to catch it.

Example sidetiq needs celluloid 0.16 but it was not checked.

Just creating a Gemfile with the gemname and running bundle install --local would be enough. If there is already a Gemfile, we can test that too.

I think gem2deb test runner should do this for all gems. We can add an option to disable it in rules.