Warn and use "oldest possible" dependency upon failure to find dependency

[Schwad]

This is in the case that there is a valid rubygem, but the date is simply too old.

The current behavior is a reversion to the most recent stable gem. After pairing with @flavorjones; I think we agreed that any user of this gem probably would prefer the oldest-available version.

However we don't want this to be done silently, so to fix this issue the plugin should:

• revert to oldest
• warn that it is reverting to oldest, with a link to the index of rubygems releases

interesting context

currently the oldest possible gem version is July 25, 2009 in the rubygems api. That is because of the rubyforge migration (I believe). I'm pairing with someone on rubygems.org tomorrow to try and rectify that, which may actually solve 30-50% of this problem too

[Schwad]

Let's link this to this rubygems.org issue

I still think there's something worth tracking here but if this gets merged it could solve the vast majority of cases here! because we'd actually be getting those pre-july-2009 gem versions. And then the PR resolving this could just be catching an edge case issue.

[Schwad]

Idea: we actually have the power to do this in the API already ; using built_at

e.g.

if gem_version.created_at == RUBYGEMS_IMPORT_DATE
  gem_version.built_at
else
  gem_version.created_at
end