Update cloudkey.gemspec to pass validation

changed README.md to Readme.md to fix validation message (I hope it's as simple as that).

example warning: 
cloudkey at /home/admin/apps/fiverr/shared/bundle/ruby/1.9.1/bundler/gems/cloudkey-72090346dfca did not have a valid gemspec.
This prevents bundler from installing bins or native extensions, but that may not affect its functionality.
 The validation message from Rubygems was:
 ["README.md"] are not files