Formula: fix class-level version accessor

When given no arguments, this should return the stable version, but it
hasn't since we stopped setting this direction in the class's @version
variable.

Library/Homebrew/formula.rb

@@ -723,7 +723,6 @@ private
     end
 
     def version val=nil
-      return @version if val.nil?
       @stable ||= SoftwareSpec.new
       @stable.version(val)
     end