gpg: add version method to retrieve current version in path

Library/Homebrew/gpg.rb

@@ -6,6 +6,7 @@ class Gpg
       gpg_short_version = Utils.popen_read(gpg, "--version")[/\d\.\d/, 0]
       next unless gpg_short_version
       gpg_version = Version.create(gpg_short_version.to_s)
+      @version = gpg_version
       gpg_version == Version.create("2.0") ||
         gpg_version == Version.create("2.1")
     end
@@ -25,6 +26,10 @@ class Gpg
     File.executable?(GPG_EXECUTABLE.to_s)
   end
 
+  def self.version
+    @version if available?
+  end
+
   def self.create_test_key(path)
     odie "No GPG present to test against!" unless available?