Adds clarification that epel is required

Resolves issue MODULES-1419
On Centos, you will get an error if epel isn't present. Clarifies this in the documentation.
Updates README with epel dependency
Issue #260
Merge pull request #260 from spuder/patch-1

Adds clarification that epel is required
Separate system and unit test gems

This commit brings the .travis.yml and Gemfile files more in line with
the other modules. This should fix the broken TravisCI builds.
Merge pull request #274 from cmurphy/fix_travisci

Separate system and unit test gems
Allow disk as a valid cluster node type

http://www.rabbitmq.com/clustering.html