Fix RedHat os version parsing & epel release

[ernestas-poskus]

@JasonGiedymin

I have found the following bugs:

• os_version_major parsing
• epel-release-7-5.noarch.rpm package not found

os_version_major parsing fails with

 TASK [ansible-mesos : debug] ***************************************************
       task path: /tmp/kitchen/roles/ansible-mesos/tasks/RedHat.yml:2
       fatal: [localhost]: FAILED! => {"failed": true, "msg": "'dict object' has no attribute u'\\\\1'"}
        to retry, use: --limit @/tmp/kitchen/default.retry

[ernestas-poskus]

https://dl.fedoraproject.org/pub/epel/7/x86_64/e/

• /epel-release-7-5.noarch.rpm is no longer available however there is new package

epel-release-7-6.noarch.rpm added 2016-04-01

[ernestas-poskus]

Same is for marathon

• os_version_major parsing

https://github.com/AnsibleShipyard/ansible-marathon/blob/master/vars/RedHat.yml#L2

Made PR AnsibleShipyard/ansible-marathon#23