[WIP] Remove inactive projects from /contributing

[duckinator]

If we're going to be pointing people at projects, we ought to be pointing them at projects that are actively maintained.

Given that, I updated the /contributing page in the following ways:

1. Removed rubygems-status link, because it's deprecated.
2. Removed rubygems-test link, because it's been untouched for >6 years.
3. Removed isolate link, because it's been untouched for >2.5 years.

I'm unsure if the following need to be updated:

1. is gemwhisperer still a thing? (cc @qrush)
2. should the link to tomlea/geminabox be changed to geminabox/geminabox?
   • tomlea/geminabox is 96 commits behind geminabox/geminabox..
3. is stickler still a thing? (cc @copiousfreetime)

Other questions:

1. is there anything else I've not mentioned that should be updated?
2. for anything that is no longer active, is there an updated variant or an actively-maintained alternative?

[hsbt]

👍

Should we archive them? see https://help.github.com/articles/archiving-a-github-repository/

[duckinator]

@hsbt it seems appropriate, but i'll let other folks handle it. 😛