Editing README to add additional dependancies on debian

The erlang version provided in the official debian repository
 doesn't handle SSL ciphers used by puppet (when used as a CA)
The more recent version offered by the erland repo (as set up
 by the erland module) alleviates this.
Merge pull request #197 from frconil/update-readme

Editing README to add additional dependancies on debian
Merge branch 'havana' of https://github.com/gwdg/puppetlabs-rabbitmq into havana