Update README name in gemspec and Rakefile

While trying to create build the gem, I got the following error message:

~~~
ERROR:  While executing gem ... (Gem::InvalidSpecificationException)
    ["README.markdown"] are not files
~~~

This is because the `README` file name was updated in fd4e5f77951f9df5aaae117dd0ba1ab524706587, but it was not updated in the gemspec.  I also noticed that the file name had not been updated in the Rakefile.  Thess patches fix both files.