Fix to detect the major release version for redhat/centos 7

In Redhat/CentOS 7, `lsbmajdistrelease` was not being set due to a minimal install not having all the required dependencies for `lsbmajdistrelease` and falling back to the default `$os_rel = 6`. This caused the puppet install to fail due to the wrong yum repo being set. As you can see below the differences in the facter output: 

```
[vagrant@localhost ~]$ facter kernel lsbmajdistrelease
kernel => Linux
lsbmajdistrelease => nil
```
compared to:
```
[vagrant@localhost ~]$ facter kernel operatingsystemmajrelease
kernel => Linux
operatingsystemmajrelease => 7
```
 
Puppet version: 3.7
CentOS version: `CentOS Linux release 7.0.1406 (Core)`

After these changes were made, I tested on CentOS 5.10, 6.5, and 7.0

This may be related to issue #445 

I am fairly new to Puppet and apologize for any guidelines that weren't followed while submitting this pull request.