instance method determine_install_method

# File railties/lib/rails/commands/plugin.rb, line 388
    def determine_install_method
      best = @base_command.environment.best_install_method
      @method = :http if best == :http and @method == :export
      case
      when (best == :http and @method != :http)
        msg = "Cannot install using subversion because `svn' cannot be found in your PATH"
      when (best == :export and (@method != :export and @method != :http))
        msg = "Cannot install using #{@method} because this project is not under subversion."
      when (best != :externals and @method == :externals)
        msg = "Cannot install using externals because vendor/plugins is not under subversion."
      end
      if msg
        puts msg
        exit 1
      end
      @method
    end