Error loading rubygems


During an operation (rspec related) that required loading rubygems
(from config/boot.rb file) I got the following error message:

~/config/boot.rb:1:in `require': no such file to load -- rubygems
    from ~/config/application.rb:1:in `require'
    from ~/config/application.rb:1
    from ~/config/environment.rb:2:in `require'
    from ~/config/environment.rb:2
    from ~/spec/spec_helper.rb:3:in `require'
    from ~/spec/spec_helper.rb:3
    from ~/spec/models/user_spec.rb:1:in `require'
    from ~/spec/models/user_spec.rb:1
    from /usr/lib/ruby/1.8/spec/runner/example_group_runner.rb:15:in
    from /usr/lib/ruby/1.8/spec/runner/example_group_runner.rb:15:in
    from /usr/lib/ruby/1.8/spec/runner/example_group_runner.rb:14:in
    from /usr/lib/ruby/1.8/spec/runner/example_group_runner.rb:14:in
    from /usr/lib/ruby/1.8/spec/runner/options.rb:132:in
    from /usr/lib/ruby/1.8/spec/runner/command_line.rb:9:in `run'
    from /usr/bin/spec:3

I looked for this error on the internet, however they all mentionned
that the problem could come from the existance of multiple versions of
ruby on the machine. Actually that was the case, so I decided to run a
fresh installation of ruby and rails using RVM. But I still have the
same problem!

Additional information:

which ruby

which rails

which gem

gem --version

ls /usr/lib/ruby
1.8 1.9.1 gems
Any idea?