Fix History.txt processing for empty LANG

When LANG=UTF-8 some versions of ruby are not able to correctly process
the History file and raise an encoding exception (notably, FreeBSD).

This ensures the History text is processed with UTF-8 encoding.

Author: drbrain

lib/rubygems/commands/setup_command.rb

@@ -124,7 +124,9 @@ def execute
                       open release_notes do |io|
                         text = io.gets '==='
                         text << io.gets('===')
-                        text[0...-3].sub(/^# coding:.*?^=/m, '')
+                        text.force_encoding Encoding::UTF_8 if
+                          Object.const_defined? :Encoding
+                        text[0...-3].sub(/^# coding:.*?^=/mu, '')
                       end
                     else
                       "Oh-no! Unable to find release notes!"