gem install で特定のversionをインストールする

$ gem install gem_name -v version_number
$ gem install gem_name:version_number

Can’t use –version with multiple gems. You can specify multiple gems with version requirements using gem install ‘my_gem:1.0.0’ ‘my_other_gem:~>2.0.0’

ref. https://github.com/rubygems/rubygems/blob/master/lib/rubygems/commands/install_command.rb