Fix error invalid byte sequence without en_US.UTF-8 locale

Because Ruby does not set `Encoding.default_external` with
non-existent locale such as `en_US.UTF-8`. Original code expects that
`en_US.UTF-8` is available locale implicitly. However, it shouldn't be
assumed.

In this case, we can use `-Eutf-8:utf-8` to specify external encoding
and internal encoding.

guides/Rakefile

@@ -21,7 +21,7 @@ namespace :guides do
     desc "Generate HTML guides"
     task html: :encoding do
       ENV["WARNINGS"] = "1" # authors can't disable this
-      ruby "rails_guides.rb"
+      ruby "-Eutf-8:utf-8", "rails_guides.rb"
     end
 
     desc "Generate .mobi file. The kindlegen executable must be in your PATH. You can get it for free from http://www.amazon.com/gp/feature.html?docId=1000765211"