I want release msgpack-ruby 0.4.8

msgpack-ruby version 0.4.7 require failed in Ruby 2.0.0.

```bash
$ ruby -v
ruby 2.0.0dev (2013-02-08 trunk 39161) [x86_64-darwin12.2.1]
$ pry
[1] pry(main)> gem 'msgpack', '0.4.7'
=> true
[2] pry(main)> require 'msgpack'
LoadError: cannot load such file -- msgpack
from /Users/yuichi-tateno/.rbenv/versions/2.0.0-rc2/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:45:in `require'
```

We fixed msgpack require probrem for Ruby 2.0.0.

I want to release msgpack-ruby 0.4.8 because many library require msgpack '~> 0.4.x'.