Fix call to deleted method

In commit a3231a03fa171d7e701150957ef4f6f74c2b6948 of the homebrew/brew
repository, the `Cask::Cask#versions` method was removed. We relied
on this method to compute the `formatted_version` we wish to display
to the user once the pinning is done.

To achieve the same result, we can simply call `Cask#installed_version`
instead.

lib/bcu/command/pin_add.rb

@@ -36,7 +36,7 @@ def run_add_pin(cask_name)
         end
 
         formatted_cask_name = "#{Tty.green}#{cask_name}#{Tty.reset}"
-        formatted_version = "#{Tty.magenta}#{cask.versions.first}#{Tty.reset}"
+        formatted_version = "#{Tty.magenta}#{cask.installed_version}#{Tty.reset}"
 
         puts "Pinned: #{formatted_cask_name} in version #{formatted_version}"
       end