Specify Rakefile encoding to keep Ruby 1.9+ happy

The Rakefile contains a multi-byte character, thus it needs an encoding or
Ruby 1.9 and newer [will refuse to process the file][ref].

  [ref]: http://blog.grayproductions.net/articles/ruby_19s_three_default_encodings
Merge pull request #138 from Sharpie/rakefile-encoding

Guts: Specify Rakefile encoding to keep Ruby 1.9+ happy