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gem install simple_form installs 1.5.2. How can install version 2.0?
You need to use the git repo. Note that the branch 2.0 is still experimental. And please use the mailing list for questions!
Sorry, but the version 2.0 is not release yet. You can use the master branch adding this line on you Gemfile
gem simple_form, :git => 'git://github.com/plataformatec/simple_form.git'
But remember it is still in development, so it is not production ready.