Update repo_name Enum for new 'beta' repo

[JayH5]

Pull Request (PR) description

Along with the APT repo update (#150), the repo name seems to have changed from testing to beta for beta releases.

See http://docs.grafana.org/installation/debian/#apt-repository

This Pull Request (PR) fixes the following issues

I haven't created an issue.

[bastelfreak]

Hey @JayH5, could you add an acceptance test for this?

[bastelfreak]

Can you take a look at the failing travis jobs?

[JayH5]

I'm not certain about the RedHat/CentOS stuff as I don't have a lot of experience with the platform. beta1 is technically the release:

[root@351a8e69d3f1 /]# yum info grafana
Loaded plugins: fastestmirror, ovl
Loading mirror speeds from cached hostfile
 * base: centos.mirror.ac.za
 * extras: centos.mirror.ac.za
 * updates: centos.mirror.ac.za
Available Packages
Name        : grafana
Arch        : x86_64
Version     : 6.0.0
Release     : beta1
Size        : 54 M
Repo        : grafana
Summary     : Grafana
URL         : https://grafana.com
License     : "Apache 2.0"
Description : Grafana

(Apologies for all the force pushing, keep noticing mess I left behind)

[bastelfreak]

Thanks!