Error while installing an older version of a gem after Sep 1st(Rails 3.0.0 release)

Question

I am on Ruby 1.8.6 and Rails 2.3.8. I am trying to install a older version of formtastic gem.

gem install formtastic -v 0.9.10

I got the following error:

ERROR:  Error installing formtastic:
        activemodel requires Ruby version >= 1.8.7.

I switched to Ruby 1.8.7 and tried to install the gem again. I got the following error:

ERROR:  Error installing formtastic:
        activemodel requires activesupport (= 3.0.0, runtime)

I have used this version of the gem with Ruby 1.8.6 before. Why am I getting this error now?

Did the release of Rails 3.0 somehow causing this behavior?

I see the same issue when I try to install paperclip.