Be able to generate doc for rspec-rails 4

[benoittgt]

With this PR:

• Generate documentation for every last release when they are available. It means we can generate documentation for rspec-rails 4 with this code without generating other gems at the moment.
• Fix a bug by not choosing beta or release candidate version like v4.0.0.rc1
• Handle major branch change with various Gemfile variations. Avoid error like:

---

For example with:

$ bundle exec rake "update_docs[4.0, 4-0-maintenance]"

We only have rspec-rails with a v4.0.0 version at the moment, so the command will generate documentation only for rspec-rails 4.0 documentation, other rspec gem will be skipped.

If we run:

$ bundle exec rake "update_docs[3.9, 3-9-maintenance]"

Every last 3.x release version of documented rspec gems will have their
documentation re-generated.

My main problem is, it not super clear from outside what the command does. Also the command link again rspec-rails to other gems in a certain way.

[JonRowe]

What about making it so it requires a gem name? Then adding an update_all_core_docs that just loops over our existing core gems?

[benoittgt]

Good idea @JonRowe. I mark this as a To Do for a new PR. It will need some refactoring to be able to use it on other command too.