Add Rake to development dependencies

On one of my installations of Ruby 1.9.3 and Bundler 1.3.5, 'bundle
exec rake' fails unless Rake is listed as a development dependency
of the project. (Interestingly, it works on another.) As Rake _is_
a development dependency of the project, list it as such.
Merge pull request #5 from jvirtanen/fixes/rake

Add Rake to development dependencies