Add rake dependency

To avoid error with Ruby 2.0 in which running `bundle exec rake` yield

/lib/bundler/rubygems_integration.rb:214:in `block in replace_gem': rake is not part of the bundle. Add it to Gemfile. (Gem::LoadError)

[Closes #2]
Merge pull request #3 from joneslee85/add-rake-into-gemfile

Add rake dependency