buildRubyGem: fixes gemspec UTF-8 loading issue

If a gemspec has UTF-8 characters in it, ruby will fail loading it with

    invalid multibyte char (US-ASCII)

This change forces the encoding to be correct, we assume everyone now
uses UTF-8.

pkgs/development/ruby-modules/gem/nix-bundle-install.rb

@@ -6,6 +6,11 @@ require 'fileutils'
 require 'pathname'
 require 'tmpdir'
 
+if defined?(Encoding.default_internal)
+  Encoding.default_internal = Encoding::UTF_8
+  Encoding.default_external = Encoding::UTF_8
+end
+
 # Options:
 #
 #   name        - the gem name