Fix RabbitMQ Installation

[jgwilson42]

This PR relates to the issue - #22

I believe that the root cause of the issue is that bintray.com is no longer available (or not providing RabbitMQ packages / binaries).

I have updated the script to use the currently documented install approach for RabbitMQ on Ubunutu.

A couple of things that I suggest baring in mind when reviewing this script:

1. I have only tried this in AWS, I assume it will work in GCP etc. but worth a test
2. This change essentially pulls a script from CloudSmith and runs it as root. Please be sure you are comfortable with the implications of this. If not, feel free to update with a different install approach.