Use ActiveSupport’s #second_to_last.

Library/Homebrew/global.rb

@@ -8,6 +8,7 @@ require "pp"
 require_relative "load_path"
 
 require "active_support/core_ext/numeric/time"
+require "active_support/core_ext/array/access"
 require "config"
 require "os"
 require "extend/ARGV"

Library/Homebrew/tab.rb

@@ -78,7 +78,7 @@ class Tab < OpenStruct
     end
 
     if attributes["source"]["spec"].nil?
-      version = PkgVersion.parse path.to_s.split("/")[-2]
+      version = PkgVersion.parse path.to_s.split("/").second_to_last
       if version.head?
         attributes["source"]["spec"] = "head"
       else